Possible approaches to JDK-8185886: Improve scrolling performance of TableView and TreeTableView

[Kevin Rushforth]

Sorry for the badly formatted html. Here it is again.

I see some progress being made on the {Tree}TableView performance issue. 
To summarize where I think we are:

There are currently 2 different approaches under review:

1. https://github.com/openjdk/jfx/pull/108 : optimization in javafx.base 
to make removing listeners faster
2. https://github.com/openjdk/jfx/pull/125 : optimization in TableView 
to reduce the number of add / removes

In addition, the following is a WIP PR that the author thinks could be a 
3rd approach:

3. https://github.com/openjdk/jfx/pull/185 : optimization in scene graph 
to avoid install treeShowing listeners on Window and Scene for all nodes

Jose has proposed a 4th approach as a comment to PR #108, and as I 
understand it, he will propose a PR shortly.

4. Don't clear the list of children in a VirtualFlow when the number of 
items changes.

So the first thing that is needed is to evaluate the approaches and 
decide which one to pursue.

Options 1 and 3 are more broad in their scope, option #2 is more 
targeted (to TableView), but within that area is a larger change. Option 
#3 would remove the (internal) treeShowing property as a generally 
available concept and only use it for controls like ProgressIndicator 
that really need it. Option #4 affects only controls that use 
VirtualFlow (ListView, TableVIew, TreeTableView), and seems not to be a 
large change (presuming we can verify that no leak is introduced).

I note that these fixes are not mutually exclusive, but I do think we 
need to settle on a primary approach and use that to fix this issue. If 
there are still performance problems after that fix, we can consider one 
(or more) of the others.

Comments?

-- Kevin