Allow delegate to customise dimmer cutout height without custom CAShape

[vpopovyc]

I had case when Drawer provided puller tip at the top of drawer body with transparent background. In result I get something like this:

I know there is an option to make custom shape, but I thought that delegating that stuff to a drawer could be a nice idea.

[ulmentflam]

@vpopovyc Thanks for your PR! At this time I am not going to merge it into master, as I don't believe this is the approach we want to take for customizing the cutout and can support your desired use case in other ways like constraining the top of your gripper to the top of the drawer instead and modifying the top inset of the drawer when .open.