slick register API for extended attribs

[jerch]

See remark in #5879 (comment)

[PerBothner]

I suggested in this comment replacing ExtendedAttrs with Markers.

@jerch commented that Markers are a "performance minefield". I don't think they have to be, if we attach Markers to LogicalLines as in the PR #5853. A difficulty is keeping the line field up-to-date - but what if we get rid of the line field? There are very few places (except about 15 tests) that currently use the line field, and it seems possible to change those.

Without a line field, keeping markers current is trivial. They don't need to be updated on reflow or changes to other lines or trimming. For rendering, it is trivial to find the set of Markers that apply to a line or that overlap a given cell. If the typical number of Markers on a given line is small (zero to one or two) then a simple linked list is simple and space efficient. Since renders and other code that checks for Markers (including ones that subsume the functionality of ExtendedAttrs) usually scan a line in order, further optimizations are possible if the markers are kept sorted (though having overlapping regions complicates that).