Optimize MessageList painting by always using system font and only allowing...

Optimize MessageList painting by always using system font and only allowing bold and italics customization

This patch removes the option to customize font family and font size in message list
and for tags. It only allows customizing bold, italics and color now. As a result of
this we only need to compute sizeHints for ThemeDelegate once and we can cache it,
because we can be sure that all rows have uniform height. The computation is very
expensive and before this patch it represented almost 50% of the overall threading
work. With this patch opening a folder in KMail is hugely faster.

REVIEW: 125153