Histogram and channels branch needs testers

[EI]

If you build from source and would like to help with a bit of testing, my
branch

https://phabricator.kde.org/diffusion/KRITA/browse/thumbnail_improvements/

is ready for wider testing. It adds histogram docker and a bit of "eye
candy":

1. New histogram docker shows current image histogram. It can be enabled
from

2. Channel docker shows thumbnails of each color channel. You can use
checkmarks to enable/disable different color channels (but not alpha
channel). Double click on a channel row should disable all other color
channels.

3. Overview docker now has higher quality thumbnail (anti-aliased).

4. All three dockers update with a slight delay (~1/4 sec) after you stop
painting to avoid interrupting paint operations.

When testing please look for new slowdowns when painting, memory leaks,
inconsistent behavior, etc.

Thank you,
Eugene
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