Cloud storage: check connection before connecting

libgit2 takes forever (a minute or so) to figure out that it can't connect
to a remote server.
So if we are using https as connection protocol, quickly check utilizing
RFCs 2324/7168 to make sure we can reach the cloud server (and not some
captive portal or something).

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

qthelper.cpp

@@ -33,6 +33,7 @@
 #include <QImageReader>
 #include <QtConcurrent>
 #include "divepicturewidget.h"
+#include "subsurfacewebservices.h"
 
 #include <libxslt/documents.h>
 
@@ -1051,3 +1052,8 @@ extern "C" bool getProxyString(char **buffer)
 	}
 	return false;
 }
+
+extern "C" bool canReachCloudServer()
+{
+	return CheckCloudConnection::checkServer();
+}