method to fill the background color in document window

[SamC]

It almost ok, except that the background color will disappear when the line just be space character.(I changed the codeblock bg color to dark pink for easy to show)

[main Alexey]

I see, it's a bug. stored it to
https://github.com/Alexey-T/CudaText/issues/5378
and I will see it later.

[main Alexey]

fixed; test beta at
https://github.com/Alexey-T/CudaText/issues/5377

[SamC]

It's ok now, thanks~