UV Editor: Add filter option to control what is visible when Draw Other Objects is enabled

Previously the editor will always try to only show UV faces with the same exact active
image or image texture, which is quite difficult to control on a production shaders, where
each material can have multiple objects assigned.

The idea of this commit is to bring option which allows to easily control what to display
when "Draw Other Objects" is enabled, so currently we can have old behavior ("Same Image")
or tell editor to show everything ("All"). In the future we can extend it with such filters
as "Same Material" and things like that.

Hopefully this will help @eyecandy's workflow of texturing.

release/scripts/startup/bl_ui/space_image.py

@@ -687,6 +687,12 @@ class IMAGE_PT_view_properties(Panel):
             sub.active = uvedit.show_stretch
             sub.row().prop(uvedit, "draw_stretch_type", expand=True)
 
+            col = layout.column()
+            col.prop(uvedit, "show_other_objects")
+            row = col.row()
+            row.active = uvedit.show_other_objects
+            row.prop(uvedit, "other_uv_filter", text="Filter")
+
         if show_render and ima:
             layout.separator()
             render_slot = ima.render_slots.active