Allow to temporarily disable updates to improve performance on bulk updates

[mar10]

(Brought up by @u-874: see #553)

Dynatree had something like tree.enableUpdate(enable) that returned the previous state, which makes it easier to restore in nested calls. It also forced a redraw on enableUpdate(true).
See http://wwwendt.de/tech/dynatree/doc/dynatree-doc.html

It would be good to have tests for it (benchmarks) so we can compare different approaches.

Instead (or additionally) to suppress rendering, we should also benchmark performance, when the tree root element is detached while adding nodes (see https://api.jquery.com/detach/)

[mar10]

Waiting for @u-874 's response to #553

[mar10]

Poor performance can be observed if we are adding many child nodes to a node that is already expanded (currently empty nodes cannot be expanded, so at least one child must already exist).

This means

1. FASTEST: add nodes to a collapsed parent and keep collapsed
2. FAST: Add nodes to a collapsed parent, then expand
3. Equally FAST: Add nodes to an expanded parent, but wrap it inside enableUpdate(false/true)
4. REALLY SLOW: Add nodes to an expanded parent

Some benchmarks for a tree table: