Add basic IME support to SDL2 video driver

src/video/video_driver.hpp

@@ -88,6 +88,11 @@ public:
 		return true;
 	}
 
+	/**
+	 * An edit box gained the input focus
+	 */
+	virtual void EditBoxGainedFocus() {}
+
 	/**
 	 * An edit box lost the input focus. Abort character compositing if necessary.
 	 */