Begin porting to PlasmaComponents3

This begins the process of porting plasma-workspace to PlasmaComponents3. At
this point, only the relatively easy work is done. Every file not ported
now has been given the reason for still using PC2 in a comment on the import
line so you can see at a glance what the blocker is by running
`grep -r "org.kde.plasma.components 2.0"` in the repo.

Everything is tested and there are no breakages or functional regressions. In
the process, a few unused imports are removed.

Here are the remaining PC3 omissions blocking what's left from getting easily
ported:
- No Highlight
- No ListItem
- No PageStack
- No ContextMenu
- No QueryDialog
- No ModelContextMenu
- TabBar has no left and right tab states
- ToolButton has no built-in method to show a menu when clicked