Simplify shortcut modifiers

Restrict shortcut modifiers to be composed of only one item each.

Before, it was possible to select a list of multiple combinations of
modifier keys, like --shortcut-mod='lctrl+lalt,rctrl+rsuper', meaning
that shortcuts would be triggered either by LCtrl+LAlt+key or
RCtrl+RSuper+key.

This was overly generic, probably not used very much, and it prevents to
solve inconsistencies between UP and DOWN events of modifier keys sent
to the device.

Refs #4732 <https://github.com/Genymobile/scrcpy/issues/4732>
PR #4741 <https://github.com/Genymobile/scrcpy/pull/4741>