Rewrote using QStandardItemModel

When trying to implement adding tags to the RSpec, I spent a fair amount of time fighting the TreeItem and NodeModel classes.
It seemed like there was an issue with the interaction between the NodeModel and TreeView but I couldn't figure it out.
Scrapping the custom model and replacing it with QStandardItem and QStandardItemModel fixed my problems.  Now adding and
removing tags appears to be working.