BaseTools: Workspace classes refactor properties

BaseTools: Workspace classes refactor properties

1) use decorators
2) also change some private functions to public when all callers are
external
3) change external callers to use functions instead of directly
accessing private data.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Jaben Carsey <jaben.carsey@intel.com>
Reviewed-by: Yonghong Zhu <yonghong.zhu@intel.com>