Simplify AssistantPopup related code

Get rid off most JS logic inside QML, make it a lot more declarative.
I'm not sure if I've fixed the losing-focus bug, but I cannot reproduce
it anymore.

REVIEW: 118819
AssistantPopup: Code cleanup
Underline warnings in yellow
ProblemHighlighter: Code cleanup
ProblemHighlighter: Use KColorScheme, too