Dock area currentChanging signal introduced(part1)

currentChanging signal helps to save previous layout state before tab will be changed

Author: Opostol

src/DockAreaWidget.h

@@ -187,6 +187,13 @@ public slots:
 	 */
 	void tabBarClicked(int index);
 
+    /**
+     * This signal is emitted when the tab bar's current tab is about to be changed. The new
+     * current has the given index, or -1 if there isn't a new one.
+     * @param index
+     */
+    void currentChanging(int index);
+
 	/**
 	 * This signal is emitted when the tab bar's current tab changes. The new
 	 * current has the given index, or -1 if there isn't a new one