Prior changes to merge the view and action menus essentially moved the
action menu items into the view menu. This was the path of least
resistance; the view has a lot of hooks on items in the view menu,
whereas the there are very few hooks on items in the action menu,
meaning the latter could be moved more easily.

However, previously the view menu was disabled for Other Locations and
the action menu wasn't. So the side effect of the changes is the
remaining menu is now disabled completely on Other Locations. There are
a couple of items that could be shown for Other Locations (e.g. New
Tab), so we still want to show a menu, but this involves some
refactoring so has been deferred until now.

This commit is the first part of the refactoring; the files view owns
the menu popover, meaning that the Other Locations view doesn't have
access to it. This commit moves the ownership of the menu popover to the
toolbar. Future commits will move the common items into the popover so
all views will show them.

https://bugzilla.gnome.org/show_bug.cgi?id=764632