Replace jQuery for hide and show statements selected by ID

[Jaifroid]

This is initial work on #916.

So far, it only deals with simple substitution of jQuery statements with their native DOM equivalents for hide and show selected by ID in app.js.

[Jaifroid]

Added uiUtil.js.

For the record, these regexes do the trick (Perl):

• Hide: $subject =~ s/\$\((['"])#([^\1\n]+?)\1\)\.hide\(\)/document.getElementById('$2').style.display = 'none'/sg;
• Show: $subject =~ s/\$\((['"])#([^\1\n]+?)\1\)\.show\(\)/document.getElementById('$2').style.display = ''/sg;

[mossroy]

I did not test but it seems to be the good approach to me (bonus point for the regexp ;-) )

[Jaifroid]

Because jQuery handles errors (e.g. if an element doesn't exist but we try to set its style property), we need to be sure to test any transient elements that we are hiding or showing if there is any possibility of their being removed from the DOM or not present (yet) in the DOM. There should only be a very small number of these (possibly only the one pushed in da80bcf). Needs careful checking.

[Jaifroid]

I tested on Firefox OS simulator and IE11 (as well as Edge Chromium), as these are most likely to be affected by removal of jQuery. No problems found.