Add descriptions for different formats in the export dialog

When choosing a format from the export dialog a two line description of
the format appears. This is helpful and prevents confusion.

Signed-off-by: Gehad elrobey <gehadelrobey@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Rework the Export dialog

The UI design looked too tight in my screen.
The explanation texts were a bit inconsistent (and in one case simply
wrong).
The explanation for the initial selection wasn't shown.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Add Subsurface XML export to Export dialog

Also be consistent about the default folder we use when opening the file
dialog.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>