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/screen/screen_ops.c

@@ -481,7 +481,7 @@ bool ED_operator_posemode(bContext *C)
   if (obact && !(obact->mode & OB_MODE_EDIT)) {
     Object *obpose;
     if ((obpose = BKE_object_pose_armature_get(obact))) {
-      if ((obact == obpose) || (obact->mode & OB_MODE_WEIGHT_PAINT)) {
+      if ((obact == obpose) || (obact->mode & OB_MODE_ALL_WEIGHT_PAINT)) {
         return 1;
       }
     }