GPencil: Refactor of Draw Engine, Vertex Paint and all internal functions

This commit is a full refactor of the grease pencil modules including Draw Engine, Modifiers, VFX, depsgraph update, improvements in operators and conversion of Sculpt and Weight paint tools to real brushes.

Also, a huge code cleanup has been done at all levels.

Thanks to @fclem for his work and yo @pepeland and @mendio for the testing and help in the development.

Differential Revision: https://developer.blender.org/D6293

release/scripts/modules/bl_keymap_utils/keymap_from_toolbar.py

@@ -199,6 +199,9 @@ def generate(context, space_type, use_fallback_keys=True, use_reset=True):
                         'WEIGHT_PAINT': "weight_tool",
                         'TEXTURE_PAINT': "image_tool",
                         'PAINT_GPENCIL': "gpencil_tool",
+                        'VERTEX_GPENCIL': "gpencil_vertex_tool",
+                        'SCULPT_GPENCIL': "gpencil_sculpt_tool",
+                        'WEIGHT_GPENCIL': "gpencil_weight_tool",
                     }.get(mode, None)
                     if attr is not None:
                         setattr(kmi_hack_brush_select_properties, attr, item.data_block)