QTAV_HAVE() macro works for VC

see wtf/Platform.h
TODO: why defined() does not work
use dllimport to enable data export
let xxx id be extern and export/import for windows

dllimport/export is required for windows.
Ids can be declared in header and define only once(in source)
register GraphicsItemRenderer
init speed in AVClock ctor
add QtAV.h for convenience. also can be used as pch