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>