appMenu: Share between top bar and app context menu
We currently use similar-but-slightly-different app menus in the top bar and in the overview.
Some of the differences are reasonable (for instance the top bar is very disconnected from the dash, so "Add/Remove Favorite" would be weird there), but even there we can still share the underlying code.
Other differences are more arbitrary, and sharing code is the best way of getting rid of them.
Conclusiong: Let's share some code :-)
Fixes #2875 (closed)