Convert Library.column to regular variable.

The only reference outside of Library.__init__ was in Main, and
it can get the column using the GtkTreeView.get_column(n) method.
Convert Library.tree_data to a regular variable

Consumers can easily get the tree model using GtkTreeView.get_model()
method.

This also removes Library.get_model() by the same rationale.
Remove Library.vbox

The one place where it was referenced was get_widgets, which can
just as easily wrap the builder.get_object call.
Remove instance attribute Library.tab

This was unused by anything, even within __init__.