GP: New Cutter, Constraints and Segment selection

This commit groups a set of new tools that were tested in grease pencil object branch before moving to master. We decide to do all the development in a separated branch because it could break master during days or weeks before the new tools were ready to deploy.

The commit includes:

- New Cutter tool to trim strokes and help cleaning up drawings.

- New set of constraints and guides to draw different types of shapes. All the credits for this development goes to Charlie Jolly (@charlie), thanks for your help!

- Segment selection mode to select strokes between intersections.

- New operator to change strokes cap mode.

- New option to display only keyframed frames. This option is very important when fill strokes with color.

- Multiple small fixes and tweaks.

Thanks to @pepeland and @mendio for their ideas, tests, reviews and support.

Note: Still pending the final icons for Cutter in Toolbar and Segment Selection in Topbar. @billreynish could help us here?

release/scripts/startup/bl_ui/space_view3d.py

@@ -245,6 +245,18 @@ class VIEW3D_HT_header(Header):
                 text=lk_name,
                 icon=lk_icon,
             )
+            
+        if object_mode in {'PAINT_GPENCIL'}:
+            if context.workspace.tools.from_space_view3d_mode(object_mode).name == "Draw":
+                settings = tool_settings.gpencil_sculpt.guide
+                row = layout.row(align=True)
+                row.prop(settings, "use_guide", text="", icon='GRID')
+                sub = row.row(align=True)
+                sub.active = settings.use_guide
+                sub.popover(
+                    panel="VIEW3D_PT_gpencil_guide",
+                    text="Guides"
+                )
 
         layout.separator_spacer()
 
@@ -3955,6 +3967,7 @@ class VIEW3D_MT_edit_gpencil(Menu):
 
         layout.menu("VIEW3D_MT_edit_gpencil_delete")
         layout.operator("gpencil.stroke_cyclical_set", text="Toggle Cyclic").type = 'TOGGLE'
+        layout.operator_menu_enum("gpencil.stroke_caps_set", text="Toggle Caps...", property="type")
 
         layout.separator()
 
@@ -5317,7 +5330,47 @@ class VIEW3D_PT_gpencil_lock(Panel):
         col = row.column()
         col.prop(context.tool_settings.gpencil_sculpt, "lock_axis", expand=True)
 
+        
+class VIEW3D_PT_gpencil_guide(Panel):
+    bl_space_type = 'VIEW_3D'
+    bl_region_type = 'HEADER'
+    bl_label = "Guides"
 
+    @staticmethod
+    def draw(self, context):
+        from math import pi
+        settings = context.tool_settings.gpencil_sculpt.guide
+
+        layout = self.layout
+        layout.label(text="Guides")
+        
+        col = layout.column()
+        col.active = settings.use_guide
+        col.prop(settings, "type", expand=True)
+                
+        if settings.type in {'PARALLEL'}:
+            col.prop(settings, "angle")
+            row = col.row(align=True)
+        
+        col.prop(settings, "use_snapping")        
+        if settings.use_snapping:
+            
+            if settings.type in {'RADIAL'}:
+                col.prop(settings, "angle_snap")
+            else:
+                col.prop(settings, "spacing")
+        
+        col.label(text="Reference Point")
+        row = col.row(align=True)
+        row.prop(settings, "reference_point", expand=True)    
+        if settings.reference_point in {'CUSTOM'}:
+            col.prop(settings, "location", text="Custom Location")        
+        if settings.reference_point in {'OBJECT'}:
+            col.prop(settings, "reference_object", text="Object Location")     
+            if not settings.reference_object:
+                col.label(text="No object selected, using cursor")
+
+        
 class VIEW3D_PT_overlay_gpencil_options(Panel):
     bl_space_type = 'VIEW_3D'
     bl_region_type = 'HEADER'
@@ -5560,6 +5613,7 @@ class VIEW3D_MT_gpencil_edit_specials(Menu):
         layout.operator("gpencil.stroke_join", text="Join").type = 'JOIN'
         layout.operator("gpencil.stroke_join", text="Join & Copy").type = 'JOINCOPY'
         layout.operator("gpencil.stroke_flip", text="Flip Direction")
+        layout.operator_menu_enum("gpencil.stroke_caps_set", text="Toggle Caps...", property="type")
 
         layout.separator()
         layout.operator("gpencil.frame_duplicate", text="Duplicate Active Frame")
@@ -5787,6 +5841,7 @@ classes = (
     VIEW3D_PT_snapping,
     VIEW3D_PT_gpencil_origin,
     VIEW3D_PT_gpencil_lock,
+    VIEW3D_PT_gpencil_guide,
     VIEW3D_PT_transform_orientations,
     VIEW3D_PT_overlay_gpencil_options,
     VIEW3D_PT_context_properties,