Improve Icon Selection/Upload

Problem/Motivation

The UI for specifying icons right now is very bare-bones: just typing the path to an icon. This could be improved with the ability to upload an icon.

Steps to reproduce

• Enable Layout Builder on a content type display mode
• Edit the Promoted Blocks list
• For each promoted block, a text field is shown at the bottom of the form

Proposed resolution

Adding the ability to upload an icon would save a lot of questions about where icons should be saved and how to find them. Although configuration management cannot save binary files, modules like Paragraphs allow the upload of SVG icons which are then just stored directly in config. For a rarely access config like a list of icons, that seems like it would be a safe approach for this module as well.

We would have to deal with the same icon being shared (such as across multiple content types), for that I would expect that icons could be keyed based on their block ID (so the same block across multiple content types would have the same icon).

Remaining tasks

User interface changes

API changes

Data model changes