Unify search GtkComboBox so it work in gtk+-2 and gtk+-3
Use GSourceFunc also for gtk+-2
Cleanup GTK_CHECK_VERSION and unify gtk+2/3 code where possible

Replace functions available in gtk+2 and gtk+3
gdk_window_get_* | since 2.24
gtk_widget_get_allocation | since 2.18
gtk_widget_get_window | since 2.14
gtk_window_get_group | since 2.10
gtk_widget_get_mapped | since 2.20
gtk_widget_get_realized | since 2.20
gdk_window_get_display | since 2.24

Remove deprecated GtkNotebookPage and check.
Remove has_separator property from gtkbuilder files