Added Widget.Tag property

Currently it is impossible to mark dynamically created widgets for their subsequent identification. The Windows Forms and WPF for this purpose have a Tag property. In this pull-request I added a Tag property into the XwtComponent.cs.
Any claims or suggestions about my edits are welcome.
The pre-pull-request discussion was here: https://groups.google.com/forum/#!topic/xwt-list/SlWKPJ7lpBQ