Implement library organisation

[shibco]

This is an issue divided into two parts but with a single goal: manipulation of the library’s tree structure within the app.

1. Implement a drag and drop UI pattern for managing files and directories. Clicking and dragging should create a transparent copy of the cell, that follows the cursor. Dropping this on another directory should complete the action.

2. Add a custom modal (issue # tbc) that asks the user for a complete directory string. In this case, this would be truncated to the library’s base uri.