Solid-compatible source code editor for solid-panes system

Extracted from the solid-panes monolithic repository.

The source code allows one to mint a new source code file and display and edit an existing one.

If the user edits a data file (such as turtle format) then the syntax is checked before they save it back.

Once selected this pane as a view of a file, then the pen icon allows to switch to editing mode, and the check mark button allows the file to be saved back.

Editing can be aborted by the cancel(X) button.