Add support for powerdevil, cinnamon, mpv, smplayer

[Lastique]

This PR adds support for:

• Blocking screensaver in KDE5 (powerdevil)
• Blocking screensaver in Cinnamon (adopted from https://github.com/iye/lightsOn/pull/23)
• Blocking screensaver when mpv or SMPlayer is running.
• Blocking screensaver when any application opens a fullscreen window. This is useful for games, when the user controls the game with a gamepad, which is not registered as a user activity.

Additionally, the PR cleans the implementation a little, e.g. to spawn less processes while operating, use canon comparison operators and adjust formatting.