Directory Tree state is destroyed on refresh

[adamk]

The current implementation of the directory tree destroys the entire tree and rebuilds it from scratch each time, resulting in a fairly high performance hit and a tree that doesn't retain it's state.

Change the tree update code to do an differential reflow (add new nodes, update existing nodes, and delete removed nodes) instead of blowing the whole thing away and doing it fresh every time.

This is a really embarrassing bug, as it makes Padre look actively buggy.

[waxhead]

The core of this hasn't changed, however Adam did say that the impact of this problem is improved with his work done to reduce the amount of refreshes being done within Padre itself.

[zenogantner]

Is this still a problem? Is it still critical?