Gives the theme editor a horizontal scrollbar

Fixes #34509 where the theme editor would push away the inspector
if something like "hseperation" is really high.
Now `set_enable_h_scroll` is true which fixes this.

(cherry picked from commit d602be077d4c540b8de0be34b6872c8ac5ea8dd0)

editor/plugins/theme_editor_plugin.cpp

@@ -634,7 +634,7 @@ ThemeEditor::ThemeEditor() {
 	ScrollContainer *scroll = memnew(ScrollContainer);
 	add_child(scroll);
 	scroll->set_enable_v_scroll(true);
-	scroll->set_enable_h_scroll(false);
+	scroll->set_enable_h_scroll(true);
 	scroll->set_v_size_flags(SIZE_EXPAND_FILL);
 
 	MarginContainer *root_container = memnew(MarginContainer);