add-bookmark-popover: Save bookmarks on both popover show and close

Due to the fact that a bookmark is added to the manager when the popover
is closed, a press on the bookmark button followed by a press on the "Remove
bookmark" button causes a crash because ephy_bookmarks_manager_remove_bookmark()
tries to find the new bookmark which has not been added yet.

Revert to the original behaviour where a bookmark is saved immediately
after pressing the star, but also save the bookmark when the popover is
closed in case the user changed something while the popover was open

https://bugzilla.gnome.org/show_bug.cgi?id=780851