Add option to set WM_CLASS

[uglym8]

Done per user request in #263

[ThomasAdam]

Hi @uglym8,

It's very unusual for the end-user to set/override the WM_CLASS property. This is because of how further properties are used by window managers, where there's assumptions about this string (such as the class name starting with a capital letter).

What are you using this proposed change for?

Incidentally, if you could, you should double-check that the default WM_CLASS property is set to Rdesktop and not rdesktop as this is in violation of the ICCCM2/EWMH, although I appreciate not part of your original PR.