Windows: Require at least Windows XP SP2

* Windows XP SP2 is 0x0502, see
  http://msdn.microsoft.com/en-us/library/aa383745.aspx
* If a module changes the Windows SDK version setting,
  this is done module wide now. So the overall behavior
  is as before. This seems to be the best compromise for
  now.
* We need at least SP2 because of the bluetooth stuff
  used in sd/source/ui/remotecontrol.
* Now, we require at least Internet Explorer 7.0. IE6
  has been outdated for a long time.
* Leave StdAfx.h file definitions, as those are Microsoft
  project specific precompiled header files.
* All local definitions of WINVER are removed, because
  the global WINVER setting makes them obsolete now.

To the relation of the three macros:

Setting _WIN32_WINNT sets WINVER and NTDDI_VERSION
automatically to the same value as _WIN32_WINNT.

WINVER and NTDDI_VERSION can be set idenpendently each
for itself.

Change-Id: Ibcc12493aae4fcaf7bcfda88be99c1b61bc326cb
Reviewed-on: https://gerrit.libreoffice.org/6496
Reviewed-by: Thorsten Behrens <thb@documentfoundation.org>
Tested-by: Thorsten Behrens <thb@documentfoundation.org>