Layout Bugzilla component update

Some weeks ago I mentioned that it might be good to update the Bugzilla components we have for Layout-related things to better reflect how the code is organised and distributed between components.

Here is a proposal for that:

https://docs.google.com/document/d/1idw9zNczbCQudgcO-OxiY3xI8YxTpTEsoHdHHjJ2ao8/edit

I have merged a few components and split a few out from existing components.  Please comment in the document if you have any suggestions, and we can discuss it in the standup meetings next week.

Thanks,

Cameron