Add XwtComponent.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've 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 and there: https://github.com/mono/xwt/pull/300.

Because of some Git issues, I'm started everything from zero and made the changes again. Sorry for any inconvience.