Make the "sort selection" tool do something.

* Handle <input> tag.
* Pass input to running process.
* Search for executable in path.
* Add "CurrentSelection" variable.

Remaining issue: ReplaceSelection output handling.

src/plugins/texteditor/texteditorplugin.h

@@ -76,6 +76,7 @@ private slots:
     void invokeCompletion();
     void invokeQuickFix();
     void updateSearchResultsFont(const TextEditor::FontSettings &);
+    void updateVariable(const QString &variable);
 
 private:
     static TextEditorPlugin *m_instance;