Allow delaying color picking
Currently if one wants to pick a color from a backdrop window, it's not possible to do it in the headerbar without fiddling with the GTK inspector in the targeted app. I don't even know what's the setting or CSS to change there TBH, though it's likely possible to disable backdrop. One way of allowing such color picking would be to delay the change to a color pick cursor by a few seconds (only when requested?), but it needs design I suppose because it's not clear how one would be indicate that there is a delay before being allowed to pick a color. Maybe a counter showing up in the headerbar would fit? Also, I wonder if the portal allows a window to change to use the color picker portal when not focused? I know some portals don't work if no parent window handle is given, or if the window is not focused, so maybe that's the case here too.