List/Tree View Improvement / GtkToolbar / GtkScrolledWindow

This is a mix of things. I fixed issues in the tree/list view although some features are still missing. I implemented GtkTreeModelFiltered that allows to filter TreeModels. I also enhanced the TreeModel interface that now has some getindex/setindex methods.

Added GtkScrolledWindow type and a basic GtkToolbar implementation. In test.jl a simple test for it is shown.