Allow the user to cancel a dive computer download

The code pretended to support this for libdivecomputer based downloads,
but it had never been hooked up when the native Uemis downloader was
implemented. When I finally decided to close that feature gap I realized
that the original code was, shall we say, "aspirational" or "completely
bogus" and therefore never worked.

So instead of just hooking up the code for the Uemis downloader I instead
implemented this correctly for the first time for both libdivecomputer and
the native Uemis downloader.

In order not to have to mess with multithreaded Gtk development I simply
opted for a helper function that fires on a 100ms timeout and have it end
the dialog without a response. This way we can run the dialog while
waiting for the download to finish, still update the progress bar and
respond in a useful manner to the user clicking cancel.

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

libdivecomputer.h

@@ -18,6 +18,7 @@ typedef struct device_data_t {
 	progressbar_t progress;
 	int preexisting;
 	gboolean force_download;
+	GtkDialog *dialog;
 } device_data_t;
 
 extern GError *do_import(device_data_t *data);