Drop support for glib < 2.36
Remove all calls to deprecated gtk_dialog_set_has_separator
Replace GtkObject with Gobject in gtk+2 build
Replace deprecated gtk widget function

The replacement functions are available since 2.18
GTK_WIDGET_HAS_FOCUS -> gtk_widget_has_focus
GTK_WIDGET_VISIBLE -> gtk_widget_get_visible
GTK_WIDGET_SET_FLAGS -> gtk_widget_set_*
Drop support for gtk+ < 2.24

* Bump required gtk+2 version
* Remove pluma-spinner
* Remove pluma-message-are
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