Don't fire SessionLoadPost event until session is fully loaded.
Bug fixin'
On session restore, maintain cwd after loading NERDTree.

There's a NERDTree option which will change the working directory to its root directory when opened. Our sessions should keep their working directory despite this option.

This change records the working directory before loading NERDTree, load it, then cd to this saved directory.
No longer try to track dirty sessions; assume always dirty.

Trying to track dirty sessions was causing more problems than it solved. It only took into account the list of buffers and window layout, ignoring all the other options than get saved with a session that may have changed. There's no good way to know what options are getting saved with the session, so for now, assume a session is always dirty.

This will cause you to always be prompted to save a session upon closing a session, but that's the trade-off.