Expose inverting icons to Python API

Some modal default Blender operators have their available hotkeys displayed in the status bar and when related hotkeys are pressed they appear "inverted", so user can see that related mode is activated (see "Ctrl" and "V" icons in the attached screenshot).

Currently this not exposed to Python API - it's possible to make those inverted for "Tab" or "Return" as they have dedicated "TAB_FILLED", "RETURN_FILLED" icons for this but for other keys Blender doesn't store the icons and instead it's changing the current theme for the button to get the effect.

So, the only current workaround is to make your "filled" Blender icons and use them from Python API.

Related - blenderartists.org/t/how-to-add-modal-modifiers/1564461