Checkerboard background for clear visualization of palette colors with transparency

[Lehonti]

The code corresponding to the feature can be examined in 4367073.

On the way I also changed StatusBarColorPaletteWidget (and related classes) to receive IPaletteService instead of PaletteManager.

I am selecting blue with an alpha of 128, and transparent. You can see the result by looking at the checkered spots:

Before this change these spots would have looked (in the case of dark mode) dark blue and very dark gray, like the background.

Note: You will notice that I'm not showing a screenshot of the old behavior. I simply can't start Pinta anymore. If I try to run dotnet run --project Pinta the command returns with no output. I had to use the cloud build in order to run my changes, and even then I had to try in another computer, because neither 3.0.3 nor the development build would run of mine. And now neither of them runs on either computer. No combination of installing/uninstalling/restarting computer/building from source/cleaning .NET solution will get Pinta to run. I don't know what's going on. For context, both are Windows machines, one of them is Windows 10 and another one is Windows 11.

Update: I got Pinta to run by deleting AppData/Roaming/Pinta. This is what it looked like before this change, but after fixing the bug that prevented changing colors in the Palette (so 3.0.3 didn't work for this):

Update: Commit 9082e84 adds this feature to the primary/secondary color swatches, too

[cameronwhite]

Looks good ,thanks!