Drag to rotate the canvas with Ctrl+Shift+Middle Mouse Button instead of Ctrl+Middle Mouse Button

Hi 👋

As of 1.0 beta 1, one can drag to rotate the canvas by holding Ctrl+Middle Mouse Button. However, when zooming and panning across the canvas, this means the artist has to press and release Ctrl every time they'd switch from zooming to panning (or they'd accidentally rotate).

I think it'd make more sense for "drag to rotate canvas" to be bound to Ctrl+Shift+Middle Mouse Button instead, as one can already scroll to rotate the canvas via Ctrl+Shift+Scroll Wheel. That way both canvas rotating operations would sit behind the Ctrl+Shift modifiers, and they wouldn't interfere with existing habits from 0.92 (Ctrl+Middle Mouse Button would pan like it used to).

For convenience, I'm pretty sure these changes would live in tool-base.cpp.

Thank you for reading. ❤