Code review comment for lp:~garyvdm/bzr-explorer/better_tree_filter

This makes the _QBrowseFilterProxyModel better use TreeFilterProxyModel. This makes the code simpler, and faster.

I've also make the auto expand more intelligent.

Unfortunately when I did this, I discovered that TreeFilterProxyModel is not as extendible as I thought. So I have made some improvements to TreeFilterProxyModel in qbzr. In order to be compatible with older versions of qbzr, I've duplicated some of the qbzr code into BE. Once we don't have to support qbzr 0.18, we can remove this.