Allow to JS-filter blocks in regions on the block layout administration page

Problem/Motivation

Using many blocks on the block layout administration page (/admin/structure/block) on large sites makes administration very hard.

As the module list for example already implements a simple javascript search to filter the results, the same would be very helpful to filter regions for blocks with the given name or category

Steps to reproduce

Proposed resolution

Implement a javascript filter which reduces the displayed block results to matches

Remaining tasks

1. Never hide the block regions.#62
2. Add a control (checkbox?) for each block region to unhide filtered blocks within that region.#62
3. Do we need a way to clear all these controls? Assuming a checkbox, how should it be labeled: perhaps "Show all" or "Show all blocks in this region". Come up with some help text (description) for the filter, mentioning the override. #62
4. The filter should "float" like the bulk actions on the node list page#62
5. Create Followup: Add a "clear" widget to the filter field. Currently the clear widget is browser based, should be for Core in general and accessible. #62
6. Provide some sort of filter status about how many blocks are filtered for each of the block regions.#63
7. Currently there is no aural feedback about he filtering results #63
8. If you keep the filtering for the string "menu" and drag the Main navigation block up past the Primary menu heading until it sticks then the region in the select field changes to Header but there is no heading called Header.#63
9. Improve the table drag icon that it reflects the possible drag directions instead vertical and horizontal all the time#3389317: Differentiate visually dragging with and without hierarchy. #63
10. Create followup: Dragging a block by the drag handle adds a * but if you change the block position via the select field no * character is shown. #63
11. Aural interface for links and buttons is sort of redundant and impossible to distinguish #63
12. Add aria-pressed to the show filtered blocks button #66
13. Add reduced motion meta query to the move to results link. #70
14. use hidden instead of filtered for the status lines (4 filtered this region) #72
15. Let the search filter behave like the bulk actions header#72
16. Not all buttons reachable by the keyboard#76
17. Indicate in the status line if there are no blocks in a block region#76
18. If a block is moved by the region select field (when row weights is active) the hidden blocks of the block region are shown#80
19. Status rows should always be right after to the block region row they describe.#80
20. Visually highlight matches (in particular if show filtered is active for a block region) #80
21. Filter box looks too prominent due to its height #80
22. Enabled and disabled blocks are highlighted differently. Plus disabled might also have a too low color contrast. #86
23. Font size and touch target for the go to results link might be too small #86
24. Adjust the micro copy of the status line to "x blocks filtered"#86
25. Make sute the filter field set and table header are displayed correctly at the top of the page with the navigation module installed instead of the toolbar #106

User interface changes

API changes

Data model changes

Release notes snippet