tabletmodemanager: properly export properties

in order to read from the properties directly (as opposed to via
org.freedesktop.DBus.Properties.Get) we need to export slots despite the
class having none (presumably the READ functions count as slots for the
purposes of qdbus?)

this makes direct calls to the interface work correctly
`qdbus org.kde.KWin /org/kde/KWin
org.kde.KWin.TabletModeManager.tabletMode`


(cherry picked from commit 3c4686ca)