when loading non-existant file, return null; check in IDE
update_status(): update URL, status bar, ...
Empty editor when loading; disable during load
Change status handling again
Render: accept scale as additional parameter
Export: make sure, that the form is complete (+ update modulekit-form)
Export: when sending request, hide only button and use as status indicator
Export: reset form when requesting
Export: show bbox in textarea; check validity
Export: check for finished job every second (instead of every 5)
Export/Render: move actual rendering to php script
Move statistics-db handling to db.php
Render: write start/stop times to database