Selection border style configuration through the settings object

[warpech]

Description

Starting from version 8.0.0 (specifically from PR #6157, it is no longer possible to set a custom border color or border width of current cell, area, fill selection using CSS. (demo of how that woked in 7.x: https://jsfiddle.net/kayw7e20/1/).

This feature was not officially supported, but we are expecting that some developers used that possibility. For a while it was listed in our docs, as @wojciechczerniak writes in #6467.

A new solution requires specifying cell, area, fill selection borders using the global settings object.

Preliminary work was done in PR #6593 (now part of PR #6157) to make sure that changes in settings object are correctly re-rendered. However, the settings object does not yet allow to specify the settings upfront and this needs to be designed and developed.

It was agreed that I will do that for 8.0.0, in a separate PR from develop.

Some preliminary discussion between me and @wojciechczerniak: https://handsoncode.slack.com/archives/D93QUS3S8/p1579606981004500

[warpech]

Proposed specification of the configuration object is presented in https://kb.handsontable.com/display/HAN/Selection+highlights. It needs feedback from @wojciechczerniak, however I will tentatively start implementing it, because it was already discussed by us on Slack.

[warpech]

Implemented in the commit 45a252f, as part of the SVG borders PR #6157