Convert fields to enum

Implementation of an action to convert fields to an enumeration.
Also changes Script.DoGlobalOperationOn.

Note a few limitations:
1. It adds a cast and parenthesis even when not necessary.
2. In the dialog, it is not currently possible to choose which fields to use. I suspect this is a bug in Xwt.Gtk and I will investigate.
3. The dialog currently does not have the MonoDevelop main window as parent, because I don't know how to get it as a Xwt control.

Related: https://github.com/icsharpcode/NRefactory/pull/213