#12 compilation warnings when using WINGs/proplist-compat.h

[Christian]

To get as fast as possible rid of the libPropList library (see bug "[ 1827383 ] libPropList dependency removed") the WINGs API that is provided for libPropList compatibility was used (WINGs/proplist-compat.h).

Unfortunately this causes some compilation warnings.

In this second step, the code should be migrated to the new API of the property list code from WINGs.