Fix typo
Ensure that the edit widget refreshes whenever the editor type changes
Improve editting of corrupt images

We now detect errors loading the image and initialise a blank canvas
instead.

This means that it's possible to open HelloThere for editting, and then
change the "type" field to "image/jpeg", and the textarea automatically
switches to a blank bitmap. Next up is making it possible to change the
size of the bitmap.
Make the bitmap editor easier to see