After much discussing here comes the new KDiskFreeSpaceInfo class to replace the old KDiskFreeSpace.
KDiskFreeSpace was originally designed as an async class which parsed the output of df.

When that changed and system calls were used, the async design did not make any sense anymore.

I also revert my previous patch and mark the KDiskFreeSpace class as deprecated.
There is no need anymore for the new API I introduced before.

KPropertiesDialog is ported to the new class.