desktop: allow update of tab widget colors at run time

Adding a new virtual function to all of these classes may seem like overkill,
but of course the idea is that likely we'd allow similar changes to all of
them.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

desktop-widgets/tab-widgets/maintab.h

@@ -64,6 +64,7 @@ slots:
 	void enableEdition();
 	void escDetected(void);
 	void updateDateTimeFields();
+	void colorsChanged();
 private:
 	Ui::MainTab ui;
 	bool editMode;