Improvements to layer info subtitles

An attempt to improve layer info subtitles.

Added: On/off checkbox changed to combobox with four styles;

• None ("off"),
• Simple (shows only opacity or blending modes that have changed),
• Balanced (shows both opacity and blending mode if either are changed; previous "on" behavior),
• Detailed (always shows opacity and blending mode).

Fixed: Handle layer types with different default blending modes. Layer types where blending mode and opacity can't be changed are no longer shown.

Thread on Krita Artists

Test Plan

Settings are in Layers docker hamburger menu.

1. Change subtitle styles, confirm opacity and blending mode show as expected.
2. Confirm that Filter Layers and Colorize Masks consider "Copy" and "Behind" respectively as the default blending mode (not shown in Simple mode, or in Balanced mode with 100% opacity set).
3. Confirm that Filter, Transparency, Transform, and Selection Masks show nothing, even in Detailed mode.

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.