w32_common: prevent display power management

Handling SC_MONITORPOWER doesn't seem to prevent the screen from
dimming. The recommended way to do this in Windows XP and Vista is to
call SetThreadExecutionState with ES_DISPLAY_REQUIRED. Windows 7 also
has the PowerCreateRequest/PowerSetRequest/PowerClearRequest APIs but
they're probably too complicated for this task.
Merge pull request #124 from rossy2401/w32-display-required

Prevent display power management in Windows