The only object flag is a type property (parenting is object type specific).
There is no way to change it after construction and there must not be. This
reduces the size of DiaObject to 96 (32 bit OS) and just needs one
indirection per object_flags_set() call.
Also remove unused DIA_OBJECT_GRABS_CHILD_INPUT - another leftover from
the unfinished "new group" stuff.

tests>test-sizeof

DiaObject: 96
Connection: 224
Element: 392
UMLClass: 952
Handle: 32
ConnectionPoint: 32