GP: Draw Mode: Add Arc Primitive

This adds an elliptical arc primitive.
Press CKEY for toggling closed/open arc.
Press FKEY key for flipping arc.

Additional changes to gpencil primitives.
Increases default edges of circle to 64.
Keymap changes to allow primitives to be drawn with Shift or Alt key.
Allow Plus/Minus key to adjust number of edges.

Missing: Toolbar icon

Differential Revision: https://developer.blender.org/D4024

release/scripts/startup/bl_ui/space_toolsystem_toolbar.py

@@ -1067,6 +1067,15 @@ class _defs_gpencil_paint:
             keymap=(),
         )
 
+    @ToolDef.from_fn
+    def arc():
+        return dict(
+            text="Arc",
+            icon="ops.gpencil.primitive_arc",
+            cursor='CROSSHAIR',
+            widget=None,
+            keymap=(),
+        )
 
 class _defs_gpencil_edit:
     @ToolDef.from_fn
@@ -1573,6 +1582,7 @@ class VIEW3D_PT_tools_active(ToolSelectPanelHelper, Panel):
             _defs_gpencil_paint.line,
             _defs_gpencil_paint.box,
             _defs_gpencil_paint.circle,
+            _defs_gpencil_paint.arc,
         ],
         'GPENCIL_EDIT': [
             *_tools_gpencil_select,