Allow using the middle mouse button to pan the image in addition to the left mouse button

**Is your feature request related to a problem? Please describe.** I can only pan with by holding down the left mouse button, but I'm used to the middle mouse button in other applications. **Describe the solution you'd like** Allow using the middle mouse button to pan the image *in addition to* the left mouse button. In addition, it would be nice to allow using the right mouse button as well (which is friendlier towards trackpad users), but the Web browser might not always allow this. Still, it's worth implementing if possible :slightly_smiling_face: I can look at implementing this feature if it's desired. **Does other service/app have this feature?** Most graphic-related applications have this feature. Here are some examples: - [GIMP](https://gimp.org) - [Inkscape](https://inkscape.org) - [Godot](https://godotengine.org)