Expand/collapse animation shows displaced child nodes when tree is scrolled (jQuery UI 1.12)

[mar10]

Expected and actual behavior

While the expand/collapse animation is running (jQuery UI 'blind' effect), the child nodes are displaced.

Steps to reproduce the problem

1. Use jQuery UI 1.12
2. Open a tree in the browser
3. Scroll down the tree (resize the browser window to a smaller height if neccessary)
4. Expand the 'examples' folder

Environment

• Browser type and version:
• jQuery and jQuery UI versions:
• Fancytree version:
  enabled/affected extensions:

[mar10]

See also #716

This is caused by a different behavior of the jQuery UI 'blind' effect implementation.
jQuery UI 1.11 wrapped the<ul> element inside a <div class='div.ui-effects-wrapper'> wrapper, while the animation is running.
jQuery UI 1.12 does not create this wrapper. Instead another element is appended after the <ul>
<ul class='ui-effects-placeholder'>.

Solution:
While an animation is running, we add a class fancytree-animating to the parent <li>.
Also a CSS rule is added:

li.fancytree-animating {
	position: relative;
}

(we also add this class to the node's <ul> directly for #716)