wayland: use mpv internal key auto-repeat handling

This removes a good chunk of code trying to recreate key repeat.
Because the wayland protocol and xkbcommon don't have an interface for
auto-repeating pressed keys.
wayland: use GetTimerMS for hiding mouse cursor

This commit remove a lot of linux specific code, like epoll. It also reduces
the complexity of the code. Instead of epoll we use poll which makes the
wayland backend more portable to other platforms.
wayland: remove input modifier variable

This variable was never used anywhere.