Fix color history and siblings position popups in multiple monitor setup
This fixes two bugs.
Bug:410959 In macOS switches the active desktop to the first desktop Krita was opened everytime a color_selector_based widget is called. (color history, mypaint color selector…)
Bug:394139 On multiple monitor setups, if any of the displays had negative coordinates. This happens because primaryScreen topLeft corner is set as origin (0,0), and cursor position uses virtualDesktopCoordinates (or global coordinates).