Added LibInput listener to toggle CapsLock

[ErikReider]

Adds a backend Dbus service which swayosd can optionally connect to which tells swayosd which special keys (capsLock, scrollLock, volume up, etc...) are being released + their LED state.

This fixes an issue that I've been annoyed with on Sway where pressing CapsLock + any other key wouldn't execute the CapsLock binding.

Todo:

• Update README with Usage
• Make sure that all of the services/rules work as expected

[ErikReider]

@jgmdev you might want to update the PKGBUILD to use meson

[simonvanderveldt]

@ErikReider Small question regarding this change, is there a way to disable trying to access the listener when using swayosd?
I don't have the listener running (I just want to use the keybindings in the config), but when I run any swayosd command it now hangs waiting for the listener

$ swayosd --brightness raise
SwayOSD LibInput Backend isn't available, waiting...

P.S. Brightness change works fine for me, can't reproduce the issue as described in #12