Enhance XML Export UI in WinForms

[ibboard]

The basic UI is in the WinForms app, but Clutch thought a few more changes were required. Rather than keep ticket:355 open, this ticket was created.

The Export form should remember the last used XSL and the last used export file. This will save quite a few clicks from the users on a new export.

(Not assigned a milestone as it is workable as it is, so could come later)