Brought back the actions that open the Properties window

Even though the Properties window is now a dockwidget that can be left
open, some people may prefer to keep it closed and only open it when
needed. The old menu items were re-introduced to make it easier to open
the Properties window in the right context.

This commit reverts parts of 480f1e245d392d52fd, 20eb8cc1de431c8f756 and
d178b4596e8504179b707617.