The datagrid needs to shuffle around some elements and their visibility

when setting up state. This was involving one more loop than needed

and some inefficient element lookups.
We now use children() where possible, instead of find(), to do the

lookups. Two for loops were also collapsed into one.
There were also some redundant lines of code that could be replaced with

pre-computed values.
Last, but not least, the new thead is hidden until we build it and are

ready to switch to it. This avoids some extra rendering.

Tested with the browser performance tools. Saw some reductions in the paint

and script execution time.
Tested that things still rendered correctly on all browsers.