GUI: Build Model(QAbstractItemModel), compatible with tree view

[tangkong]

Build qt model that displays a Entry objects in a QTreeView.

Some things to consider

• How best to enable lazy loading of Entry objects. Does the model make lazy requests? Needs client to be configured for this.
  • What hooks exist? qt naturally has a QAbstractItemModel.canFetchMore method that is typically used to determine when more rows can be added (on scroll event or otherwise)
  • We also want to fetch information when the tree view tries to expand its children. This might better exist in the item when .child is called? Then perhaps the model would have to know about a Client.
• Tree model and table model are different in nature (indexes operate with different assumptions)
• This ideally works for any Entry, to enable displaying subtrees in child pages

[tangkong]

Initial development in #38

[tangkong]

I'll close this with #69, there are other tickets for the sub-items and I'm growing weary of tracking meta-tickets