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, which
required adding gvdb_table_write_contents_async(), which I upstreamed to
GVDB. (It was added to Epiphany in the previous commit.)

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
3 jobs for mcatanzaro/#336 in 2 minutes and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #334310
flatpak

00:01:57

 
  Deploy
passed #334311
review

00:00:35

manual #334312
allowed to fail manual
stop_review