Support for collections and requests

[danirod]

This is my take on #11.

The goal is to allow the user to create collections, which will have endpoints inside. It should be possible to create, open, save and close collections, and for each collection, the application shall let the user manage folders and requests.

Tasks and subtasks:

• Create collections.
• Open existing collections by pointing at the metadata file.
• Close existing collections
• Update information about collections.
• Support for requests and folders (to-do...)

Known issues:

• Pressing Enter on the New Collection dialog should submit the form.

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Here is an updated screenshot with the state of this feature.

Here we see the list of opened collections.

By pressing the [Add] button, you can create a new collection.