buttonrebinds: permit binding any mouse button

This makes it possible to swap the "middle" and "back" buttons, for example.

Scratching on some itches:

• https://discuss.kde.org/t/regular-mouse-button-remapping-in-kwin/13573
• https://bugs.kde.org/show_bug.cgi?id=475879

Example of this in the kcminputrc config file:

[ButtonRebinds][Mouse]                                                                                                                                                                                                                                                                                                   
MiddleButton=MouseButton,275                                                                                                                                                                                                                                                                                             
ExtraButton1=MouseButton,274

Native button codes are needed. I expected these to follow the definitions from libinput, so I'm very confused as to why "back" is not 278. I determined by trial and error that it is 275. I'd be curious to find out why.

Next on the to-do list is to make this config per-device instead of global.