Mod search window to not close with return key

Currently, OgreKit's search window closes automatically when search next command is invoked using return key. We changed this behavior to close the search window only if return key is pressed with shift key. Otherwise just perform search next and the search window remains open.

This fix is based on the following blog article by @aki-null:
http://blog.aki-null.net/blog/2011/10/21/coteditor/