LibGUI: Unindent selected text on shift+tab press

Selected text is unindented when Shift+Tab is pressed. Select text,
indent it with Tab, then unindent with Shift+Tab.

Userland/Libraries/LibGUI/TextDocument.h

@@ -271,4 +271,16 @@ private:
     TextRange m_range;
 };
 
+class UnindentSelection : public TextDocumentUndoCommand {
+public:
+    UnindentSelection(TextDocument&, size_t tab_width, TextRange const&);
+    virtual void undo() override;
+    virtual void redo() override;
+    TextRange const& range() const { return m_range; }
+
+private:
+    size_t m_tab_width { 0 };
+    TextRange m_range;
+};
+
 }