Fix T63125: Gpencil: bones cannot be selected in weightpaint mode

Some underlying functionality was not ready for greasepencil:
- BKE_modifiers_get_virtual_modifierlist (now introduce dedicated BKE_gpencil_modifiers_get_virtual_modifierlist)
- BKE_modifiers_is_deformed_by_armature
- checks in drawing code
- checks in (pose) selection code

A couple of changes to make this work:
- `eGpencilModifierType_Armature` has to be respected (not only `eModifierType_Armature`)
- `OB_MODE_WEIGHT_GPENCIL` has to be respected (not only `OB_MODE_WEIGHT_PAINT`) --  (now use new `OB_MODE_ALL_WEIGHT_PAINT`)
- `gpencil_weightmode_toggle_exec` now shares functionality from `wpaint_mode_toggle_exec` -- moved to new `ED_object_posemode_set_for_weight_paint`

This patch will also set the context member "weight_paint_object" for greasepencil (otherwise some appropriate pose operators wont work when in weightpaint mode)

Reviewed By: campbellbarton

Maniphest Tasks: T63125

Differential Revision: https://developer.blender.org/D8483

source/blender/editors/include/ED_object.h

@@ -352,6 +352,11 @@ void ED_object_mode_generic_exit(struct Main *bmain,
                                  struct Object *ob);
 bool ED_object_mode_generic_has_data(struct Depsgraph *depsgraph, struct Object *ob);
 
+void ED_object_posemode_set_for_weight_paint(struct bContext *C,
+                                                    struct Main *bmain,
+                                                    struct Object *ob,
+                                                    const bool is_mode_set);
+
 /* object_modifier.c */
 enum {
   MODIFIER_APPLY_DATA = 1,