Previously, we were relying on working history to track the
most recent cursor position. That is rather prone to failure
so instead we'll use the gvfs backend to store
metadata::libide-position with a value like "line:offset" to
restore the cursor. We will also accept just "line" which will
result in 0 being used for the offset.

Line and offset use a zero index.

This does require that gvfs is installed to work correctly,
and jhbuild has been updated to include that as a dependency.

Long term, we should investigate a metadata manager of sorts
to be attached to IdeContext so that metadata operations can
be blocked (asynchronously) during context unloading.