- move Properties to its own file

Signed-off-by: hearts <hearts.zhang@gmail.com>
- dummy

Signed-off-by: hearts <hearts.zhang@gmail.com>
- add Properties.cs

Signed-off-by: hearts <hearts.zhang@gmail.com>
- remove depth parameter. it's not used

Signed-off-by: hearts <hearts.zhang@gmail.com>
- remember layout pos in measureoverride
- only arrange in arrangeoverride

Signed-off-by: hearts <hearts.zhang@gmail.com>
- add Size.eq member

Signed-off-by: hearts <hearts.zhang@gmail.com>
- refine some code

Signed-off-by: hearts <hearts.zhang@gmail.com>