Panes should have labels/titles

[hturner]

Comment from @ajrgodfrey

Apart from the terminal input window, there are no labels/titles explaining what the different panes are.

[ajrgodfrey]

This specifically affects the script window (at first) and any files opened later.

[georgestagg]

As of 7df7964 the four main panes are labelled with role='region' and e.g. aria-label="Editor Pane".

I've also reworked the labelling of elements throughout the page, including updating the tabs at the top of the Editor Pane so that each open file tab is labelled with its filename. The editor should now also update its aria-labelledby attribute to point at the tab corresponding to the currently open file.

Hopefully, this makes the app easier to navigate via screen-reader, but do let me know if there are any other significant elements I've missed that require labelling.

(It will take a few hours for the webR website to update, but the changes should be available by tomorrow.)