Draft: Tree-view

This MR is a feature implementation of #41 (closed) .

TODO

• Add entry & other UI elements to toggle & define tree-view behavior
• Modify the model to support tree-view in details view
• Tree navigation with arrows; similar to tree-view-sidepane
• Image previews disappear on selection
• Handling Tree Model in search and location bar
• Pressing F5 removes selection in subfolders.
• Split-view/new tab now respects the last view active view's tree view setting if the view was Details-view.
• Thunar Critical on exit.