Poor performance of mail viewer

[vogel76]

Looks like QTextBrowser/QTextEdit has a performance problem. It appears on mail containing ~10 replies/forwards (html size is ~30kB)

Problem is especially visible while opening standalone maileditor window with mail having set some attachement since because of buggy code (improper use of MailBox::previewImages inside MailEditorMainWindow::LoadMessage) document contents are loaded twice.

To create such message send new mail to own identity and reply/forward it ~15 times. Opening/displaying such message will take ~30s (measured on quite fast machine in release build)

[PaulEU]

For now, it's about three times faster, but it's still too slow.