Introduce master displays

Bit of a stub for the moment.

we need to introduce a new ConfigEntity for master displays, and implement their inheritance into normal displays. master displays are mostly just a rehash of normal displays, excepting that we have to be a little smarter about how we order the blocks in the regions. instead of just recording a weight, we need to keep track of information like "stick to top" or "stick to bottom" of region; this information will be necessary to intelligently resolve block ordering in a regular display when inheriting from the master.

initially, we'll restrict inheritance of a master display to a normal display that's using the same layout plugin. we can explore expanding that later (maybe in contrib), but attempting to do layout remapping AND block ordering resolution at the same time would be daunting.