UV: Enable 'Set Snap Base' feature

Just like for objects in 3D View, UVs can also benefit from the
`Set Snap Base` feature (key `B`).

source/blender/editors/transform/transform.cc

@@ -710,7 +710,7 @@ static bool transform_modal_item_poll(const wmOperator *op, int value)
     case TFM_MODAL_EDIT_SNAP_SOURCE_OFF:
       return false;
     case TFM_MODAL_EDIT_SNAP_SOURCE_ON: {
-      if (t->spacetype != SPACE_VIEW3D) {
+      if (!ELEM(t->spacetype, SPACE_VIEW3D, SPACE_IMAGE)) {
         return false;
       }
       if (!ELEM(

source/blender/editors/transform/transform_mode_snapsource.cc

@@ -194,10 +194,13 @@ void transform_mode_snap_source_init(TransInfo *t, wmOperator * /*op*/)
   }
 
   t->mode_info = &TransMode_snapsource;
-  t->flag |= T_DRAW_SNAP_SOURCE;
   t->tsnap.target_operation = SCE_SNAP_TARGET_ALL;
   t->tsnap.status &= ~SNAP_SOURCE_FOUND;
 
+  if (t->spacetype == SPACE_VIEW3D) {
+    t->flag |= T_DRAW_SNAP_SOURCE;
+  }
+
   customdata->snap_mode_confirm = t->tsnap.mode;
   t->tsnap.mode &= ~(SCE_SNAP_TO_EDGE_PERPENDICULAR | SCE_SNAP_INDIVIDUAL_PROJECT |
                      SCE_SNAP_INDIVIDUAL_NEAREST);

source/blender/editors/transform/transform_snap.cc

@@ -1788,7 +1788,9 @@ float transform_snap_increment_get(const TransInfo *t)
 
 void tranform_snap_source_restore_context(TransInfo *t)
 {
-  snap_object_context_init(t);
+  if (t->spacetype == SPACE_VIEW3D) {
+    snap_object_context_init(t);
+  }
   snap_multipoints_free(t);
 }