set formfactors for applets that shouldn't be on phone

Many applets shouldn't actually appear on the phone, like devicenotifier, the battery applet, the systray and so on Populate all desktop files of the plasmoids with the proper formfactors. on the desktop we keep showing everything. on the phone most of those applets will be filtered out

Also take into account of the formfactors when populating the available categories, wo we don't have the risk of seeing empty categories