Change credits presentation of splash image

• Use version without signature
• Add "Artwork by: Tyson Tan" as QLabel instead
• Make "Artwork by:" text translatable (second commit, shouldn't be cherry-pick to 5.0?)

CC @tysontan (don't know if this actually pings?)

Test Plan

Check splash screen

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• n/a 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.