Screen/Windows: Allow activation of GUI windows via mouse click.

kPsychGUIWindow's return MA_ACTIVATEANDEAT in response to mouse
activation events, as opposed to MA_NOACTIVATEANDEAT for other windows.

-> Clicking an inactive GUI window will activate it.
-> Also apply minor fix to DWM timestamping in the rare cases where it can
   be used and doesn't work, e.g., compositor active on a single display,
   single onscreen window setup (where it can be used), but inside a Windows-7
   VM, where it fails with invalid values on VirtualBox. We double-check timestamp
   for validity and return a failure code on invalid timestamp, so fallback path
   can be used.
-> Various dead code removal.

-> Screen for 32/64-Bit Matlab on Windows rebuilt and tested.

Fixes #156