Minor changes to splash screen and about dialog

Made some minor changes to the splash screen code, also simplifying it a bit by using a QLabel instead of painting text manually.

Test Plan

• Verify that the splash screen still shows on launch.
• Verify that the splash screen shows up centred on the active monitor, instead of always on the primary monitor.
• Verify that the About dialog shows fine.

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.