Convert system apps to "launcher-plugin"-apps for faster startup

[rkunze]

The new folder based launcher introduced in 6c4daa7 allows running specially prepared apps in the same python process as the launcher itself, which cuts down the launch time for these apps from approximately five seconds to less than one second - although at the risk that a defective app can kill or block the launcher.

Because system apps are usually not too complicated, pretty well tested, and can benefit vastly from the enhanced startup times, convert all system apps to this new "launcher-plugin" style.

[rkunze]

Implemented in commits d4abd20...e10469f