the main objective was to fix the bootstrap modules in framework builds.
bootstrapped modules which "borrow" headers from "proper" modules can
specify this in a clean way now.
a side effect of this is that the bootstrap-dbus module now has its own
syncqt call.

most includepath-related setup from qt_module_pris.prf was moved to
qt_module_headers.prf.

Change-Id: Ie0d8192cfac1a8cdae0ddd0bc0cd8c3092b1e85b
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>