Windows: Only set the touch flags if the window is not already registered

It is possible for there to be a HCBT_CREATEWND hook which can set the
touch window flags already while the window is being created. Therefore
we want to defer to those settings instead as they should take precedence.

Change-Id: If8dcbd34db2b3bbbfb1bc36731665fb17fb87c24
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>