Desktop: support the Eyedropper outside the app window

Future improvement after #2340: support color picking anywhere onscreen using a custom borderless window following the cursor displaying the current color and magnified pixel area visualization. Except on Wayland, which apparently doesn't allow this.

Start by reading issue #2340 and the existing desktop Eyedropper implementation. Investigate whether a borderless window can follow the cursor and display the current color with a magnified pixel visualization, while checking the stated Wayland limitation. Done means color picking works outside the app window on supported systems without requiring support where Wayland prevents it.

Scritto dal modello di indicizzazione a partire dal testo della issue.