rqt_image_view: Adding the possibility to publish mouse events.

Allow for some interactivity by capturing and publishing mouse clicks on the displayed image. I didn't see any possibility to do that from within rqt right now except for a rather complicated approach [recently discussed in ROS answers](http://answers.ros.org/question/56185/creating-interactive-image-displayin-rviz/).

The mouse click location is published as [geometry_msgs/Point](http://ros.org/doc/api/geometry_msgs/html/msg/Point.html) with [0,0,0] representing the top left corner and [1,1,0] representing the bottom right corner.

The topic name is chosen automatically based on the subscribed image topic by default but it is possible to choose your own.

This is only a small extension so I didn't consider putting it into a separate plugin. That might be a good idea though, if this added functionality might cause confusion.