Like every other spinbox slider, rightclicking will allow input, and thus
setting to 0 will reset canvas rotation.
This is a rewrite of https://phabricator.kde.org/D2518
With this, the 4 and 6 buttons, the touch docker buttons, the view menu entries
the pop-up palette widget, the shift+space, I never want to hear again that
Krita lacks options to rotate the canvas.