[feature] Allow for pan and zoom with the mouse/trackpad to move the (infinite) canvas around

[mattrobineau]

I was going to ask to have zooming added because it's hard to annotate accurately when you have a huge screenshot (like 2 screens in my case) and I found this was already discussed. The issue where it is mentioned (#49) is for something else and I would prefer this feature (pan/zoom) doesn't get lost.

The cool thing would be that it brings a ton of improvements

• if something is hidden behind the toolbars, you can just move until its visible
• you can draw in and outside the image (infinite canvas)
• you can zoom in to work on a very small part of the image
• you annotate very small or very big stuff