Make ephy_bookmarks_manager_save_to_file_async() actually async

Currently it looks like an asynchronous function, but it actually does sync I/O, which is bad.

This also requires also making ephy_bookmarks_export() async, as well as gvdb_table_write_contents(). I'll attempt to upstream the GVDB changes separately.

It also requires adding GCancellable parameters where required, to avoid introducing new crashes. This means we need a way for external code to access the EphyBookmarkManager's cancellable or we would need to get rid of the warn_on_error convenience callback.

Also, remove ephy_bookmarks_manager_load_from_file(), since it's only one line long and only used in one place.

Finally, rename the functions according to Carlos's feedback.

Fixes #336 (closed)