GPv3: "Set Active Layer" operator and menu

Resolves #113915 .

Ports the `layer_active` operator to set the active layer from an index.
Adds the menu to set the active layer from a list.

Pull Request: https://projects.blender.org/blender/blender/pulls/115606

scripts/presets/keyconfig/keymap_data/blender_default.py

@@ -4575,6 +4575,9 @@ def km_grease_pencil_paint(_params):
          {"properties": [("mode", 'INVERT')]}),
         ("grease_pencil.brush_stroke", {"type": 'LEFTMOUSE', "value": 'PRESS', "shift": True},
          {"properties": [("mode", 'SMOOTH')]}),
+
+        # Active layer
+        op_menu("GREASE_PENCIL_MT_layer_active", {"type": 'Y', "value": 'PRESS'}),
     ])
 
     return keymap
@@ -4623,6 +4626,9 @@ def km_grease_pencil_edit(params):
         ("grease_pencil.cyclical_set", {"type": 'C', "value": 'PRESS',
          "alt": True}, {"properties": [("type", "TOGGLE")]}),
 
+        # Active layer
+        op_menu("GREASE_PENCIL_MT_layer_active", {"type": 'Y', "value": 'PRESS'}),
+
         # Context menu
         *_template_items_context_menu("VIEW3D_MT_greasepencil_edit_context_menu", params.context_menu_event),
     ])