Add a class in orbisgis core to convert a layer into image stream
Export map editor tool in Orbisgis view
Add DPI input
Add icon and fix layout issues.

I redid the InputPanel class's layout using MigLayout. I also removed
the grouping since it wasn't used anywhere and made the code rather
complicated to read.
Add confirmation for overwrite
Do some translations
Merge pull request #519 from nicolas-f/feature/export-mapeditor

MapEditor export image tool

* Create MapImageWriter, taken from WMS server in order to easily convert a layer into an image stream.
* Add a new tool in MapEditor to export the current rendered map into an external PNG or TIFF file.
Replace X11, headless exception by a test and log.trace
Merge pull request #527 from nicolas-f/fix/headless-error

Replace X11, headless exception by a test and log.trace