Move resetting of current file out of clear_dive_file_data()

This is the only case where C-code sets the current file.
Remove this call for a better separation of C-backend and
C++-frontend parts.

There were four callers of clear_dive_file_data(). Two of them
would call set_filename() anyway. For the remaining two add an
explicit call to set_filename().

This commit fixes a bug introduced in commit b3901aa8f9:
The cloud-online menu entry was still enabled after "closing" the
cloud storage.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/mainwindow.cpp

@@ -746,6 +746,7 @@ void MainWindow::closeCurrentFile()
 	/* free the dives and trips */
 	clear_git_id();
 	clear_dive_file_data();
+	setCurrentFile(NULL);
 	cleanUpEmpty();
 	mark_divelist_changed(false);