Feature Request: allow dragging with middle mouse button / add drag tool

[KR155E]

When working on a large image, or simply on a high zoom level, and the whole viewport is covered with pixels, the only way to move around an image is by using the mouse wheel, since it is not possible to click to the side of the image and move around by dragging the mouse. To solve this, I propose to:

1. Allow dragging the view with the middle mouse button, as other image manipulation programs do
2. Add a drag tool that allows to click anywhere to grag the image around, perhaps BrushTool.NONE could be repurposed for this