per-monitor DPI handling with MS Windows clients

[Antoine Martin]

Blocked by #56, X11 doesn't have per-monitor DPI data either.. so not very useful in the short term.

But we can get per-monitor DPI information:

$ from comtypes.client import CreateObject
o=CreateObject('WbemScripting.SWbemLocator')
s=o.ConnectServer('.', 'root\\cimv2')
res=s.ExecQuery('SELECT * FROM Win32_DesktopMonitor')
print([(x.Properties_['DeviceID'].Value, x.Properties_['PixelsPerXLogicalInch'].Value, x.Properties_['PixelsPerYLogicalInch'].Value,) for x in res])
[(u'DesktopMonitor1', 96, 96)]