[meta] Miscellaneous UI issues for the Layout Builder module

Problem/Motivation

In the process of reviewing #2922033: Use the Layout Builder for EntityViewDisplays, I noticed a number of small UI issues that could be improved. Some of these are specific to Layout Builder; others are just exposed by the module and have other underlying causes.

I'm just going to attach a bunch of annotated screenshots for now in case other reviewers come across the same things.

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9. There should be some sort of visual indicator of the region you're about to drop a block into, at present it can be difficult to get items in the right region, especially when the block is wider than the region. We should be able to use the jQuery UI API to set a background colour on the current drop target

Proposed resolution

See invisionapp https://projects.invisionapp.com/boards/KH3EPJMFNWR4Z

Remaining tasks

User interface changes

API changes

Data model changes