Make adding variable support to textcontrol easier.

Add convenience method in variable chooser and move magic constant to
the place where it's used.

Change-Id: Ie93acb98e13efcacb77d1a478df927a11b16926b
Reviewed-by: Daniel Teske <daniel.teske@digia.com>

src/plugins/coreplugin/variablechooser.h

@@ -56,6 +56,9 @@ public:
     explicit VariableChooser(QWidget *parent = 0);
     ~VariableChooser();
 
+    static const char kVariableSupportProperty[];
+    static void addVariableSupport(QWidget *textcontrol);
+
 protected:
     void keyPressEvent(QKeyEvent *ke);
     bool eventFilter(QObject *, QEvent *event);