Move device selection in its own sidebar

Instead of having a list of checkable buttons, this creates a left sidebar that lists all of the available devices. It works as follows:

• When there's no gamepad, it displays the usual placeholder:

• When there's only one gamepad, it only displays the gamepad:

• When there's two gamepads or more, and the window is big enough, it displays a sidebar:

(I don't have two gamepads, so just pretend there are two here.)

• When there's two gamepads or more, and the window is not big enough, it has two different views for the selection and gamepads: