Uses unit aware spinboxes in some strategical places, especially imagesize and image offset plugins.

Summary:
Changes may seem's minor but it's a necessary step toward being able to do unit conversion on the fly in the math spinboxes.
Plus it bring interessants like new units (vw, vh) and the ability to change active unit from the keyboard.

Ref T1621

Reviewers: Krita, dkazakov

Subscribers: rempt

Maniphest Tasks: T1621

Differential Revision: https://phabricator.kde.org/D5124