Allow mouse buttons to be key modifiers

Using extra mouse buttons (button4, 5, etc) as modifiers would make it convenient to add many custom keybinds, which make use of easy to reach keys such as A, G, etc; while at the same time keeping the default layout for convenience and compatibility with tutorials, etc.

Right now only keyboard keys can be used as modifiers. It is possible to bind the action to a mouse button, and then use a key as a modifier, but in that case, existing binds take priority and the modifier is ignored in some contexts, and if mouse input is primary, we are sometimes subject to special dragging behavior, and the "Release Confirms" setting which is not always ideal.

For example, I want to move the cursor as if it was an object (transform.translate, with cursor_transform=True). An ideal binding in this scenario would be the letter G, with Button4 as a modifier. The current alternative is to bind it to Button4 with G as a modifier, but if I have any selected elements, it will trigger the standard move on that element and ignore Button4. Also, even if I have no objects selected and the command triggers properly, binding Button4 as 'Press' will make it so I have to drag while holding down the button. Binding it as 'Release' or 'Click' are workarounds but they introduce a bit of delay.