Fix #290: Mouse-only image viewing

https://github.com/BestImageViewer/geeqie/issues/290

The mouse can now be used to change image, and zoom and pan image.

Single middle-click toggles the mouse wheel function between "change
image" and "change zoom".
In split-view quad mode each pane can be clicked, allowing Connected
Zoom to made on any combination of panes.

This function cannot be used with "Mouse wheel scrolls image" or
"Navigation by left or middle click on image" options.