wrap any plain text column headers in a <span>

[telamonian]

As discussed with @texodus, this PR changes the creation of column header nodes such that any text is wrapped up in a <span> element before getting added. As opposed to now, where any text is just included as a raw text node.

[telamonian]

This PR also fixes an API consistency issue w.r.t. what is a valid value for a column_header entry. Previously, you could pass an html string such as "<span></span>" and it would automagically get rendered into actual html nodes before getting added to the column_header. On the other, if you tried passing in eg an actual live <span> element as a column_header, it would not get handled correctly and you would instead end up with a raw text node containing nonsense (something like "[HTMLSpan object]").

On the other hand, row_header values have the exact opposite handling (<span> element works, "<span></span>" wil result in a raw text node). This PR brings the handling behavior of column_header values in line with the current handling of row_header values.

All of which is to say that we should probably consider doing a v0.3.0 version bump before releasing this change, just in case anyone is relying on the exactly wonky details of the current column_header value handling.

[telamonian]

All of the unittests now pass locally. A bunch of tests needed fixes w.r.t. the fact that column-headers are now rendered slightly differently. Most fixes were trivial, except for the sort-handling in the perspective example. The previous mousedown handler was unable to deal with clicking on a <span> child of a <th> column-header, and could only handle clicks directly on the <th> element itself. This is now fixed

@texodus Can you please review?

[texodus]

Thanks for the PR! Looks good!