A typedef sal_uLong      BitmapChecksum;
has been added to include/vcl/checksum.hxx

Wherever needed sal_uLong and sal_Int32 has been substituted with
BitmapChecksum.

A BITMAP_CHECKSUM_BITS constant equal to the amount of bits used by the
BitmapChecksum type has been defined and used in
`GraphicID::GetIDString` (vstools/source/graphic/grfcache.cxx).

Change-Id: I74bd285089e58a8b18c06233d75b87023c7bf31b