feature request: add modifier to mouse key bindings

[vincentaxhe]

waybar now have 'on-click' 'on-click-right' 'on-click-middle' 'on-scroll-up' 'on-scroll-down' five actions, could waybar support modifier to extend its actions, I found lavalauncher can do it by 'command[alt+mouse-right] = some command', it's better to integrate to waybar.