Load Wintab button mapping only once.

Summary:
This prevents Krita from losing its Wintab button map, at the cost of
requiring Krita to be restarted (or else a new option to be added
allowing settings to be reset) when users change settings in the Wintab
driver.

BUG: 359561
BUG: 349422

Reviewers: Krita:_next

Maniphest Tasks: T958

Differential Revision: https://phabricator.kde.org/D1174