Improve color picking

[KoBeWi]

This PR makes use of the new screen_get_pixel() method for ColorPicker. It allows to pick the color anywhere on the screen(s), with a caveat that it only works on supported platforms (unsupported have the picking button disabled).

I used a little hack suggested by @Sauermann to detect clicks outside the main window. I spawn a tiny 1x1 popup and wait until it disappears (which happens when you click anywhere). The color reading is done inside internal process. Pressing Escape will cancel picking and restore previous color.

Fixes #74048

EDIT:
After some feedback, I left the old picking code for platforms that don't support screen capture. It works e.g. on Android, so no reason to remove it I guess.

[Sauermann]

I noticed, that this PR will remove picking for non-desktop-platforms like Android. Perhaps the previous method should be kept for these platforms?

[KoBeWi]

I tested on Android and it seems to be working 🤔
I can restore it, I'm a bit worried about code bloat tho.

[bruvzg]

I used a little hack suggested by @Sauermann to detect clicks outside the main window. I spawn a tiny 1x1 popup and wait until it disappears (which happens when you click anywhere).

There's an issue with this approach, other apps can receive click and take focus when picking color, but I'm not sure how to solve it. On Windows, the global input hook used to close popups will consume the click event, but on other platforms it's only works with other windows of the same app.

[KoBeWi]

Ok I restored the old method of color picking. It replaces the new one on unsupported platforms.

There's an issue with this approach, other apps can receive click and take focus when picking color, but I'm not sure how to solve it.

I wouldn't worry about that too much. GIMP for example has the same issue.

[akien-mga]

Thanks!