Summary:
This is desirable because it lessens the confusion of having "Canvas Input Settings" and "Shortcut Settings." Users can see at a glance that both pages are available. I think it looks nicer, too.

Class renaming and documentation updates can be done in a separate patch.

Reviewers: #krita:_next, dkazakov

Reviewed By: #krita:_next, dkazakov

Subscribers: dkazakov

Differential Revision: https://phabricator.kde.org/D1121