Freeze file system model during editing

Freeze file system model during editing

If a file system event arrives while GtkFileChooserWidget is asking the
user to edit the name of a newly created folder, the file system model
will drop the row with the editable cell, and the user will have to
start from scratch.

This makes creating new directories impossible inside a directory with a
file currently being downloaded, for instance, and it's really unhelpful
to the user because the editable row simply disappears.

We already have a mechanism in place to freeze the file system model, so
we can reuse it between the add_editable() and the remove_editable()
calls.

https://bugzilla.gnome.org/show_bug.cgi?id=729927