online-accounts: Don't show spurious "No online accounts configured"

Programmatically deleting or inserting a row into a GtkTreeModel
implementation does not change the selection on the GtkTreeView even
if it is in browse mode (see GTK+ documentation). So, we need to
connect to the model's row-deleted and row-inserted signals and adjust
the selection ourselves.

Fixes: https://bugzilla.gnome.org/682175