Fix visually glitchy Create button in element picker

The Create button was being unduly enabled for a fraction of
second while editing the content of the filter text.

This commit is candidate for a revision to current stable
release.

src/js/epicker-ui.js

@@ -458,7 +458,6 @@ const onCandidateChanged = function() {
  const filter = filterFromTextarea();
  const bad = filter === '!';
  $stor('section').classList.toggle('invalidFilter', bad);
- $id('create').disabled = bad;
  if ( bad ) {
  $id('resultsetCount').textContent = 'E';
  $id('create').setAttribute('disabled', '');
@@ -767,7 +766,7 @@ const showDialog = function(details) {
 
  dialog.querySelector('ul').style.display =
  netFilters.length || cosmeticFilters.length ? '' : 'none';
- dialog.querySelector('#create').disabled = true;
+ $id('create').setAttribute('disabled', '');
 
  // Auto-select a candidate filter