add POLO objects to registry? eq. hs.geometry

[muescha]

          FWIW, lua only code can add its metatable to the registry so `hs.getObjectMetatable` works with the following code:

debug.getregistry()[userdata_tag] = metatable

Then, hs.getObjectMetatable(userdata_tag) will work; it also means that we can assign the metatable from C/Swift side code, even though the metatable is pure lua code.

Originally posted by @asmagill in #3541 (comment)

so shout I open an PR Request to add this to the hs.geometry file?

debug.getregistry()['hs.geometry`] = geometry

PS: and if there are other objects without a registry also to them?