ThemeEditor: What-If enhancements

Current ThemeEditor:

What-if with facility to see Tray, SerenityMenu and Tooltip

What-if with facility to see Ruler/Gutter

What-if with facility to see SyntaxHighlighting per Theme

What-if with facility to see RubberBand Fill

Then this will be one awesome ThemeEditor indeed. :^)

I agree that these would be good things to be able to see, namely the tab highlight color, which I find to be lacking in the current theme-editor. :yakplus:

A move in the right direction, https://github.com/SerenityOS/serenity/pull/13937 is quite an exciting development :^)