app-window: show_properties() take current_item into account

If we already have an item marked as current, simply go directly to
properties view, ignoring selection.

https://bugzilla.gnome.org/show_bug.cgi?id=738761
app: Make open_in_new_window() public

https://bugzilla.gnome.org/show_bug.cgi?id=738761
app: Make delete_machines_undoable.message, optional

Make message arg of delete_machines_undoable method, nullable and
optional.

https://bugzilla.gnome.org/show_bug.cgi?id=738761
Add ActionsPopover

Add a popover to present a item-specific actions menu that includes:

* Open in new window
* Favorite
* Pause
* Delete
* Properties

https://bugzilla.gnome.org/show_bug.cgi?id=738761
collection-view: Disallow selection_mode from the view

If the view is requesting selection mode, it means user has clicked
right-mouse button and we now want to show a context menu on
right-click.

https://bugzilla.gnome.org/show_bug.cgi?id=738761
collection-view: Show context menu on right-click

Show ActionsPopover when user right clicks on an item.

https://bugzilla.gnome.org/show_bug.cgi?id=738761
collection-view: Show context menu on menu key

Show ActionsPopover when user presses menu key when keyboard cursor is
on an item.

https://bugzilla.gnome.org/show_bug.cgi?id=738761