Add a listenner on the image exporter ui to update image ratio according selected combobox item
Clean up
Fix bad copy-paste
Add the patch to use the document listenner
Fix width/height listeners in export map dialog
Combine two 'keep ratio' choices into one.
Replace combo box by a "Keep ratio" checkbox.
Add "Reset extent" button
Merge pull request #538 from ebocher/feature/mapexporter

Add height/weight listeners to the image exporter gui. This PR integrates all fixes done by @agouge. Thanks to him.