Include reference images in screen color picker

This enables the screen color picker to pick from reference images like the color picker tool does. In addition, I found an issue when having documents in overlapping subwindows, it should now pick from the correct document instead of the first one that contains the cursor position.

Should fix bug 411816

Test Plan

• Configure krita to use subwindows for documents (instead of tabs)
• Open or create two or more documents and use reference images
• make some subwindows overlap
• make sure the screen color picker (from foreground/background color dialog or SVG text editor) really picks the color it's supposed to

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.