Fix a crash when opening a folder, and another one when trying to open
a newly created folder.
It was not safe to modify a GModelSelection while it's being iterated over.
Fixes#536.
In the event where you want to find the index of a deeply-nested path
with a GFileSystemModel that hasn't yet traversed most of that path, it
is possible for a false negative failure to occur. This failure is
caused by the GFileSystemModel incorrectly bailing out of the search
when it hits the first unseen path segment that is not at the very end
of the path.
This patch fixes this problem by reifying the intermediate nodes during
that search and traversal process.
This is the same as calling FileSystemPath(foo).string(). The majority of
clients only care about canonicalizing a path, so let's have an easy way
to express that.
