[Linux] Crash on startup when run in Wayland

[baconwaifu]

This is a trivially-worked-around subset of #74 that can be fixed by simply not registering the external capture hotkeys when running under wayland (which is trivially detectable via app.platformName() == "wayland") and probably popping up a "we know this is broken" dialog pointing out that external capture is broken on Wayland, but everything else works fine.