Bugzilla

This is done by adding a new entry `SelectionType::eHighlight`.
In contrast to the existing selection types, highlight selections can occur more than once
and are identified by their `highlightName` (i.e., `::highlight(foo)` in css).
This made some changes necessary to `nsFrameSelection`,
which now uses an additional hash map of highlight selections,
as well as to several methods dealing with getting style properties for selected text
in `nsTextFrame`, which now in addition to the `SelectionType` enum need the highlight name
as well.

Depends on D163008

Obsolete. For development of initial version see Bug 1803355.