Add tree filtering through proxy stores

This PR adds new ListStoreFilter and TreeStoreFilter stores, which can be used to filter other stores (IListDataSource, ITreeDataSource). For this purpose they wrap around an other store and pass a filtered version to a view (similar to Gtk implementation). A filter function can be specified, which will be called for every node/row to evaluate whether to filter (hide) it or not.

Exaples for a filtered ListView and TreeView are included.

Backends:
Gtk: native implementation using Gtk.TreeModelFilter
Wpf: works out of the box
Mac: uses default Xwt backends (both included), needs #426 (!)

Other changes:
* depends on / includes #428, #429
* Xwt: ITreeDataSource / IListDataSource implementations moved to Xwt.*StoreBase classes
* Gtk: ITreeDataSource / IListDataSource implementations moved to *StoreBase classes