Add GimpPickButton implementation for Windows

Merged Luca Bacci requested to merge lb90/gimp:add-gimp-pick-button-win32 into master
  • Works on the entire screen, not just the GIMP window
  • Works with multiple monitors across different DPI scales
  • Active windows can be switched using ALT+TAB
  • User can press ESC at anytime to exit color-picking mode

Fixes #8506 (closed)

Edited by Luca Bacci

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