kcms/mouse: Use CMake kcmutils_add_qml_kcm() for the plugin target
This removes a few lines of code and more importantly, changes the resource prefix from "/" to "/kcm/kcm_mouse/" to avoid potential conflicts with any other ui/main.qml that System Settings might load.
CC @nicolasfella, @ahiemstra & @davidre who've all been recently involved in such MRs