When waiting for initialization of the WebView2 component in the Edge
implementation, OS messages are processed by peeking the queue and
performing a display.readAndDispatch() in case a message is present. The
call to OS.PeekMessage() does, however, already perform some processing
of the messages. In fact, Edge is not supposed to process all kinds of
OS messages at all (like paint or input events) but should rather only
process the WebView2 initialization callback message, arriving as a
posted OS message. This change thus limits the message peeking inside
Edge to only process the message type PM_QS_POSTMESSAGE.

Fixes https://github.com/eclipse-platform/eclipse.platform.swt/issues/1789