Add project name to Tropy URLs

[inukshuk]

At the moment all URLs generated point to the 'current' project. In order to generate URLs for multiple projects, we should add a name to project file mapping to Tropy. Every time a project is opened Tropy can update the mapping.

The name, if not given, can be derived from the project name or it could be a short id. It has to be unique and users may want to customize it in the project settings.

• Add project name/id
• Generate a sensible default
• Store name/id file mapping
• Update mapping when a project is opened
• Use name/id in URLs
• When opening an URL, prompt for the project file if the name/id is not known