Add a Novel Tree View

[vkbo]

The project tree panel can easily be wrapped in a tab widget allowing alternative ways to browse the project. One such alternative is to add a tree view of the novel structure like the one that exists in the Outline, but a simplified one that is easier to use when just editing files.

The tree view could be as simple as a tree of the titles, like the first column of the Outline view, and the word count like in the project tree. A third column can be the point-of-view character, but the third column should perhaps be by user choice.

This feature idea arose from the discussion in #498 and parallel to #542.

[vkbo]

This feature has been merged into the dev branch.