[Umbrella] Problems with visual representation of selection in table cells

[Piotrek Koszuliński]

1. On IEs empty cells' selection is invisible. IE renders only text selection.
2. On FF there's a similar problem like on IE, with a difference that cells can be selected as a whole but only by mouse.

The full solution would be a fake table selection - similar to fake element selection which we have for non-editable elements. Partial solutions may also be possible.

Sub tickets: #8830, #12043.