Promote simple const variables to constexpr

Aleutbeo Narequested to merge
aleutbeo/krita:fix/constexpr-simple-variables into master

Promotes many simple const variables to constexpr status to "enforce its const-ness" strongly. It also prepares for any future functions / objects / datatypes also promoted to constexpr-status.

This PR does not change any working functionality of code already in the repo, just strongly enforces "constexpr-ness".

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