fix DownloadDialog behavior

The DownloadDialog behavior was broken in a way it allows the user
to make changes on the dialog while the download is happening.

Also, clicking on "Cancel" breaks/hangs the UI sometimes, as libdivingcomputer
doesn't always cancels the download right away. That's a bug that
still needs to be fixed.

Signed-off-by: Danilo Cesar Lemes de Paula <danilo.eu@gmail.com>