LibGUI: Indent selected text on tab press

If selected text is less than a whole line, usual delete/replace takes
place. Otherwise, if the selected text is a whole line or spans
multiple lines, the selection will be indented.

Userland/Libraries/LibGUI/TextEditor.h

@@ -151,6 +151,8 @@ public:
     void select_current_line();
     virtual void undo();
     virtual void redo();
+    bool is_indenting_selection();
+    void indent_selection();
 
     Function<void()> on_change;
     Function<void(bool modified)> on_modified_change;