mapfishapp - implementing #754

Adding the possibility to set the FileDescriptor's save file.
mapfishapp - file formats as JSON

Adding some methods to access the supported geo file formats (depends
whether or not OGR is available).
mapfishapp - fixing programming error

Avoid having to instantiate a daughter class to guess which file formats
are available, removing useless methods related to the available format
support (managed by another classes).
mapfishapp - using common access to supported geo formats
mapfishapp - upload / download geo file (#754)

First naïve implementation of the download feature, all the security is
based on the fact that only zip files and xml derivatives can be used as
geo files. This might deserve a better hardenning to avoid security
flaws, but should address the feature request for now.

This seems not really functional with geotools implementation, so it
might need a gdal/ogr-enabled mapfishapp.

Test: mainly manual, might need to improve / adapt test suite.