Add ability to export only selected dives in worldmap exporter

This patch adds the ability to export selected dives only in the worldmap
exporter. After Miika added the export dialog in commit 7dc642860def
("Implementing export dialog") and exporting only selected dives became a
choice while exporting.

Signed-off-by: Gehad elrobey <gehadelrobey@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Shortcuts for the exporting window.

Add the ability to close the exporting window, also Quit Subsurface with
this window in front, for consistency.

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