QmakeProject: Add PRECOMPILED_HEADER as a source for headers

Also remove files from PRECOMPILED_HEADER if needed.

Task-number: QTCREATORBUG-10621
Change-Id: Ife1f8f0c3219ad51fbd4bf1ef48fb82c404cb2a6
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>
Reviewed-by: Daniel Teske <daniel.teske@digia.com>