[TESTERS NEEDED] Input: allow multi-button assignment

[Megamouse]

• Allows you to assign multiple keys/buttons to the same virtual buttons by pressing shift while clicking on a PushButton.
• fixes Feature request: Possibility to map, one or more key to a single button #5192

TODO:

• Implement in evdev handler
• Tweak GUI interaction

[Yuuyatails]

This update is also useful when using controllers like the DualSense Edge.

[ElTioRata]

Sorry for bumping up this but I believe that as a user I shouldn't search for this PR to know how to do this. An extra note should be written in the GamePad Settings description (just like there's one for binding mouse movement).

[ElTioRata]

I also found out that if you bind a key and a mouse button to a controller button, the key will be override by the mouse button. So for example, if you bind Left Click and F key to Cross, F won't work and only the Left Click will able to press Cross.

[Megamouse]

Open an issue then.