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)