Remove using namespace Gtk; from headers

This causes issues if the header is included early on
in particular a conflict with gdkx.h
The reference to ‘Window’ is ambiguous `Gtk::Window` vs [X11]Window.

libs/gtkmm2ext/gtkmm2ext/cell_renderer_color_selector.h

@@ -26,8 +26,6 @@
 
 #include "gtkmm2ext/visibility.h"
 
-using namespace Gtk;
-
 namespace Gtk {
 	class ColorSelectionDialog;
 }

libs/gtkmm2ext/gtkmm2ext/cell_renderer_pixbuf_multi.h

@@ -30,8 +30,6 @@
 
 #include "gtkmm2ext/visibility.h"
 
-using namespace Gtk;
-
 namespace Gtkmm2ext {
 
 class LIBGTKMM2EXT_API CellRendererPixbufMulti : public Gtk::CellRenderer

libs/gtkmm2ext/gtkmm2ext/cell_renderer_pixbuf_toggle.h

@@ -27,8 +27,6 @@
 
 #include "gtkmm2ext/visibility.h"
 
-using namespace Gtk;
-
 namespace Gtkmm2ext {
 
 class LIBGTKMM2EXT_API CellRendererPixbufToggle : public Gtk::CellRenderer