This is a step towards being able to just choose the "Drivers Editor"
from the editor selector, and have everything displayed, ready to use.
Currently there are a few problems to resolve still:
1) This update callback doesn't run when changing the mode via the
editors selector in the UI. (Note: This problem also affects the
Timeline vs Dopesheet, causing a lot of initialisation work to be
skipped)
2) There's some missing redraw/refresh here to cause the properties
region to actually show. You need to resize the window for that to
happen now.
b081436d1c