Makes tree accessible

[cmcculloh-kr]

fixes #1964

[swilliamset]

why i don't see unittest being used anywhere

[swilliamset]

I only see $element used once at https://github.com/ExactTarget/fuelux/pull/1984/files?#diff-e5768895bd9d9f525527077130cf278bR70

It could be replaced with

this.$element.on('focus', function setFocusOnTab () {
	var $tree = $(this);
	focusIn($tree, $tree);
});

making this assignment unnecessary.

[swilliamset]

372b0eb
Thanks for cleaning up the eslint but the commit message doesn't reflect the scope of the work done. Please include some note stating where the eslint clean up focused.

fixes most eslint warnings and all errors in tree tests

[interactivellama]

• Needs visual focus indicator SLDS currently has:

.slds-tree__item a:focus {
  outline: 0;
  text-decoration: underline; }

• Shift+tab isn't working (might need less of a keyboard trap). This should just work based on tabindex.
• ul role="tree" node needs either aria-label or aria-labelledby
• selected items need aria-selected="true"
• tree needs aria-multiselectable unless it is single selection. multi should be default.
• if tree is single select, aria-multiselectable needs to be false. We've handled this in the past through the dataSource I think. I'm not sure how to expose with the API. a dataSource option might be good. I'm not sure the best way to do the labels and the selection ability on the top level tree node.

[cmcculloh-kr]

Working through @interactivellama's feedback now.

• Needs visual focus indicator SLDS currently has
• ul role="tree" node needs either aria-label or aria-labelledby
• selected items need aria-selected="true"
• non-selected selectable items need aria-selected="false"
• tree needs aria-multiselectable unless it is single selection. multi should be default. (Fuel UX default is single select. Implementors are free to set multi-select to true...)
• if tree is single select, aria-multiselectable needs to be false
• Shift+tab to leave the tree (and tab to previous item) isn't working. "Shift" is probably being swallowed up by the keyboard event listener.

[cmcculloh-kr]

Note that the tree markup changes slightly with this PR adding aria-* attributes and tabindex to many of the nodes in tree. It's a non-breaking change.