Refactor popup menu handling

Use gdk_event_triggers_context_menu() in button-press-event handlers.
Use GDK_BUTTON_PRIMARY instead of 1 where appropriate.
Remove last uses of GtkAction
Track main window state

This will be used to save and restore the window dimensions, and whether
the window is maximized.
Save and restore main window state
Move tag area to EtTagArea object
Move file area to EtFileArea object
Drop explicit call to gtk_main()
Remove unused code when closing preferences dialog
Remove the cancel button from the preferences

As settings changes are applied instantly with GSettings, a cancel
button is not useful.
Move dialog setting saving to application window
Implement hiding of file header widget

Show the file header information if the file-show-header GSetting is
true.
Avoid warnings when reading and writing ID3 tags

https://mail.gnome.org/archives/easytag-list/2014-July/msg00000.html