Draft: window: Add to Starred action

Introduce an item Add to Starred to the pathbar context menu and add an action for it. When the item, actually a button, is clicked the directory that is currently open is starred.

This is a draft because one specific behaviour is happening and since this is my first time I don't know whether it's expected or not. When inside the same directory on two different windows and bookmarking one of them, the other gets automatically updated, both the sidebar and the pathbar action. I think this implementation for the starred action should be able to do the same... however that is not working.

Could the fact that I'm running under a container and using dbus-run-session nautilus for the two different windows be the reason even though the bookmark works?

This closes #3190.