Multi-Object Editing

This adds initial multi-object editing support.

- Selected objects are used when entering edit & pose modes.
- Selection & tools work on all objects however many tools need porting
  See: T54641 for remaining tasks.

Indentation will be done separately.

See patch: D3101

source/blender/editors/uvedit/uvedit_intern.h

@@ -51,6 +51,8 @@ void  uv_poly_center(struct BMFace *f, float r_cent[2], const int cd_loop_uv_off
 /* find nearest */
 
 typedef struct UvNearestHit {
+	/** Only for `*_multi(..)` versions of functions. */
+	struct Object *ob;
 	/** Always set if we have a hit. */
 	struct BMFace *efa;
 	struct BMLoop *l;
@@ -66,14 +68,23 @@ typedef struct UvNearestHit {
 bool uv_find_nearest_vert(
         struct Scene *scene, struct Image *ima, struct Object *obedit,
         const float co[2], const float penalty_dist, struct UvNearestHit *hit_final);
+bool uv_find_nearest_vert_multi(
+        struct Scene *scene, struct Image *ima, struct Object **objects, const uint objects_len,
+        const float co[2], const float penalty_dist, struct UvNearestHit *hit_final);
 
 bool uv_find_nearest_edge(
         struct Scene *scene, struct Image *ima, struct Object *obedit,
         const float co[2], struct UvNearestHit *hit_final);
+bool uv_find_nearest_edge_multi(
+        struct Scene *scene, struct Image *ima, struct Object **objects, const uint objects_len,
+        const float co[2], struct UvNearestHit *hit_final);
 
 bool uv_find_nearest_face(
         struct Scene *scene, struct Image *ima, struct Object *obedit,
         const float co[2], struct UvNearestHit *hit_final);
+bool uv_find_nearest_face_multi(
+        struct Scene *scene, struct Image *ima, struct Object **objects, const uint objects_len,
+        const float co[2], struct UvNearestHit *hit_final);
 
 /* utility tool functions */