GPencil: Merge GSoC curve edit mode

Differential Revision: https://developer.blender.org/D8660

This patch is the result of the GSoC 2020 "Editing Grease Pencil Strokes
Using Curves" project. It adds a submode to greasepencil edit mode that
allows for the transformation of greasepencil strokes using bezier
curves. More information about the project can be found
here: https://wiki.blender.org/wiki/User:Filedescriptor/GSoC_2020.

release/scripts/modules/bl_keymap_utils/keymap_hierarchy.py

@@ -191,6 +191,7 @@ _km_hierarchy = [
     ]),
 
     ('Grease Pencil', 'EMPTY', 'WINDOW', [  # grease pencil stuff (per region)
+        ('Grease Pencil Stroke Curve Edit Mode', 'EMPTY', 'WINDOW', []),
         ('Grease Pencil Stroke Edit Mode', 'EMPTY', 'WINDOW', []),
         ('Grease Pencil Stroke Paint (Draw brush)', 'EMPTY', 'WINDOW', []),
         ('Grease Pencil Stroke Paint (Fill)', 'EMPTY', 'WINDOW', []),