Add size hints to tablet tester widget (for better dialogue size)

The tablet tester dialogue’s ‘preferred’ size is being overridden by KoDialog, resulting in a very small dialogue box, with a tiny painting area for testing the tablet. Now we add a sizeHint for this area, so that the dialogue automatically gets a reasonable size.

The width and height values for the sizeHint was chosen so that they correspond to the size the widget would have had if KoDialog didn’t override the preferred window size.

Test Plan

Settings → Configure Krita → Tablet settings → Open Tablet Tester

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.