toolbar: Stop creating GtkToolItems to contain toolbar elements

Since it's no longer a GtkToolbar we can just add the widgets to the header
bar without the tool item container.
ev-zoom-action: Keep a reference of the GMenu

This used to work because GTK+ was leaking the menu.
ev-zoom-action: Allow the zoom action to be smaller

Override get_preferred_width to ignore the natural size of the entry.