Allow direct data sources to be defined as strings

We should allow custom data sources to be defined, such as Turtle or JSON-LD, without it having to exist on a specific URL.

For this, the drop-down list after "Choose datasources" should show a clear button to "Add data directly", for which a dialog should be opened to set the datasource name, media type, and contents.
The media type should be selectable from a drop-down list of known media types in Comunica.
After saving the contents, the data should be queryable together with the other datasets.
This data source should also show up in the text box, and be editable again after clicking on it.

More than one such direct data source should be possible to add.

These direct data sources should be saved/loaded in the URL state as well.

Start at the "Choose datasources" drop-down and trace how datasource selection, editing, querying, and URL state are currently handled. Define the dialog flow for datasource name, media type, and contents, including multiple direct sources and known Comunica media types. Done means direct sources can be added, queried with other datasets, edited, and saved and loaded through the URL state.