Remove duplication of blending mode and category names

This makes all blending mode names and blending mode category names to be translated only in KoCompositeOpRegistry.cpp, using i18nc with the existing context string when possible. The exceptions are COMPOSITE_IN and COMPOSITE_OUT which are not registered in KoCompositeOpRegistry and had only been using i18n without context string. All other duplicated blending mode names without a context has been removed by this MR.

Can also be cherry-picked to 5.0.

Test Plan

• Check that blending mode names are properly displayed and translated.
• Make sure that no "Got null display name for composite op" warnings are printed.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.