Add UnregisterClass().
Refactor conversion of the WNDCLASS and WNDCLASSEX structures.

We will also be converting into the opposite direction.
Add ExtTextOut() and TabbedTextOut().

Message and class stuff is still too hard (as in "do I really need a hash
table to elegantly work around this crappy design because urgh, this is still
C" hard), so here's another few easier functions.

I would have also added PolyTextOut(), but apparently, no one has ever used
that function, so I don't have any test case. Judging from its assembly code
in Windows 7's GDI32.DLL, it also does a lot more things than Wine's simple
implementation might suggest...