Use standard SDL in Windows (remove custom API)

[historic_bruno]

Via Philip in IRC (referring to #907):

It might be best to just try retiring wsdl and switching to standard SDL,
which would hopefully handle this stuff automatically
...
It should be pretty straightforward to set up (disable CONFIG2_WSDL
and provide the necessary DLLs etc) - the tricky bit is just
testing that everything still works (cursors, fullscreen, multi-
monitor, Atlas, etc)

Relevant code is source\lib\sysdep\os\win\wsdl.cpp