[gui] remove GTK2 support

.gitignore

src/plugins/gui/GSourceView.2.ml

src/plugins/gui/GSourceView.2.mli

src/plugins/gui/GSourceView.ml

src/plugins/gui/GSourceView.mli

src/plugins/gui/debug_manager.ko.ml

src/plugins/gui/debug_manager.mli

src/plugins/gui/debug_manager.ok.ml

src/plugins/gui/design.ml

src/plugins/gui/dgraph_helper.mli

src/plugins/gui/dgraph_helper.no.ml

src/plugins/gui/dgraph_helper.yes.ml

src/plugins/gui/dune

src/plugins/gui/gtk_compat.2.ml

src/plugins/gui/gtk_compat.3.ml

src/plugins/gui/gtk_compat.mli

src/plugins/gui/menu_manager.ml

src/plugins/gui/property_navigator.ml

src/plugins/gui/wutil.ml

src/plugins/impact/gui/register_gui.ml