Allow easier setting of background color for TextEdit

[bircni]

• Closes Allow easier setting of background color for TextEdit #5183
• I have followed the instructions in the PR template

[github-actions]

Preview available at https://egui-pr-preview.github.io/pr/5203-patch15
Note that it might take a couple seconds for the update to show up after the preview_build workflow has completed.

[lucasmerlin]

Is there a reason why one would often need to change the background color of TextEdits?
Someone could argue that if we have this functionality for the background, we should also add functions to customize the font, size, padding, etc... I think this could get out of hand quickly.
If something like this is desirable be useful I think we should come up with some way to generalize this and add this to all widgets (and I think then this is related to #3284)

[AlexanderSchuetz97]

@lucasmerlin
I require this for 2 reasons:
Really highlight text edits in red if they contain invaluid data that the human must change.

I use gray background for every other textbox because with white background users didnt even realize the textbox exists until they moved their cursor over them.

@bircni
Thank you for implementing this.

[lucasmerlin]

I talked with @emilk and we decided that since extreme_bg_color is kind of obscure it makes sense to add this until #3284 is implemented

[bircni]

Okay! I will also add a comment to refer to the issue you mentioned

[lucasmerlin]

Thank you!