Grab and Move - allow mouse buttons as modifiers

### Description of the new feature / enhancement

It would be very useful if the new Grab and Move feature supported the use of the fourth/fifth mouse buttons (back/forward) as modifier keys, in addition to Alt/Win.

### Scenario when this would be used?

In my experience with AltSnap, using the mouse buttons as modifiers makes it even easier and simpler to use this tool. You don't need to move the other hand to the keyboard.

### Supporting information

AltSnap documentation: https://github.com/RamonUnch/AltSnap/wiki#activate-with-click

Start by locating the Grab and Move implementation and its current Alt/Win modifier handling. Trace how mouse input is processed, then verify that the fourth and fifth back/forward buttons activate the feature as modifiers without breaking the existing behavior.

Written by the indexing model from the issue text.