Validate table selection POC approach.

[jodator]

Provide a description of the task

This task considers finishing the basic table selection concept and plugin. As the plugin itself can be unusable at this stage the feature will have its own integration branch (although switch master to master is desirable).

However the engine part of this PR might be integrated to master if changes do not break anything.

[jodator]

The table selection POC has two PRs:

1. The engine part PR: I/3202: Enable multi block selection. ckeditor5-engine#1817

   It introduces:

   • selection post-fixer changes (to allow multi-range selection)
   • MouseObserver has added another event types: 'mousemove', 'mouseup', 'mouseleave' alongside 'mousedown'. Could be moved to another type of observer (dunno the name as possible DnD would use such events also).
   • a SelectionRendererObserver that stops selection rendering during selection drag changed to fake selection
   • view Renderer changes - stop rendering selection changed to fake selection
   • minor set of tests (some are failing).

   The solution needs to be triaged to ensure I didn't forget about anything as pausing the selection rendering subject comes and goes (AFAIR last time it was mentioned together with IME changes). After using fake selection mechanism the selection rendering is not needed. It turned out that I was using it wrongly before. The key is to use 'low' priority when setting the fake selection to prevent other selection change listeners from mangling with the selection.

   Next steps:

   • Create MouseObserver PR to the engine (can be merged to master any time)
   • Create selection post fixer changes PR that enables multi-range selection (it may be postponed)

2. The table part PR: I/3202 ckeditor5-table#228:

   It introduces:

   • TableSelection plugin - manages selection handling for mouse over a table - needs tests
   • MergeCells command - base implementation - might be extracted to own PR/branch and work to be finished later on
   • changes to some commands (failing tests after changes in multi-range selection) - might need fix together with TableSelection PR
   • TableEditing changes tableCell to isObject instaed of isLimit in the schema.
   • TableUI uses MergeCells command instead of current 4
   • manual test for table selection

   Next steps:

   • create minimal tables PR (without UI or commands). Done here: I/6113: Base table selection plugin ckeditor5-table#230.
   • move MergeCells command to own branch, describe work needed to finish the task: done here: Table selection handling: Merge cell split command #6123 (comment).

3. Related issues

   • Display all selection ranges ckeditor5-inspector#68