Make APP_ID match real application ID

Created by: hadess

The application advertises itself as io.github.pwr_solaar.solaar through its appdata file, so name the application this way too.

This fixes this warning in Flatpak:

Failed to register: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: org.freedesktop.DBus.Error.ServiceUnknown

I went with io.github.pwr_solaar.solaar as io.github.pwr.solaar doesn't seem like a correct reverse DNS name.