GPv3: Copy and Paste operators for copying strokes and points

This PR implements the Copy and Paste operators for GPv3. The operators
are available in Edit Mode. The Copy operator copies the selected strokes/
points to a clipboard. The Paste operator pastes the  strokes/points from
the clipboard to the active layer.

Keyboard shortcuts:
- `Ctrl`-`C` for copy
- `Ctrl`-`V` for paste
- `Ctrl`-`Shift`-`V` for paste in the back (behind all existing strokes)

Pull Request: https://projects.blender.org/blender/blender/pulls/114145

scripts/startup/bl_ui/space_view3d.py

@@ -5831,6 +5831,11 @@ class VIEW3D_MT_edit_greasepencil(Menu):
 
         layout.separator()
 
+        layout.operator("grease_pencil.copy", text="Copy", icon='COPYDOWN')
+        layout.operator("grease_pencil.paste", text="Paste", icon='PASTEDOWN')
+
+        layout.separator()
+
         layout.menu("VIEW3D_MT_edit_greasepencil_showhide")
         layout.operator_menu_enum("grease_pencil.separate", "mode", text="Separate")
         layout.operator("grease_pencil.clean_loose")
@@ -8220,6 +8225,12 @@ class VIEW3D_MT_greasepencil_edit_context_menu(Menu):
             col = row.column(align=True)
             col.label(text="Point", icon='GP_SELECT_POINTS')
 
+            # Copy/paste
+            col.operator("grease_pencil.copy", text="Copy", icon="COPYDOWN")
+            col.operator("grease_pencil.paste", text="Paste", icon="PASTEDOWN")
+
+            col.separator()
+
             # Main Strokes Operators
             col.operator("grease_pencil.stroke_subdivide", text="Subdivide")
             col.operator("grease_pencil.stroke_subdivide_smooth", text="Subdivide and Smooth")
@@ -8256,6 +8267,12 @@ class VIEW3D_MT_greasepencil_edit_context_menu(Menu):
             col = row.column(align=True)
             col.label(text="Stroke", icon='GP_SELECT_STROKES')
 
+            # Copy/paste
+            col.operator("grease_pencil.copy", text="Copy", icon="COPYDOWN")
+            col.operator("grease_pencil.paste", text="Paste", icon="PASTEDOWN")
+
+            col.separator()
+
             # Main Strokes Operators
             col.operator("grease_pencil.stroke_subdivide", text="Subdivide")
             col.operator("grease_pencil.stroke_subdivide_smooth", text="Subdivide and Smooth")