When inserting a new page, we should scroll the canvas so we’re at the top left

[maltenuhn]

Problem
If you insert a new route, we navigate to it (good) but if you'd previously scrolled down the canvas to edit an existing, longer, page, you don't see the new page (because it's a good 1000 pixels to the north).

Solution
When inserting a new page, scroll or reset the canvas so that the top left edge is at a usable spot. To save time on premature perfection, this should be hardcoded and not eg menu-width dependent, I suggest hardcoding this so the top left of the scene (not the scene label) shows up on the screen at 380px east and 120px north. (NB it might be easier to set the 0,0 coordinate of the canvas to that point - if so, that's likely preferable!)

Result should look roughly like this: