[trojita] /: Merge changes from topic 'bug-404211'

[Gerrit Code Review]

Git commit 681c99cf2429137f4462bf61232c4bb8a5a363cf by Gerrit Code Review, on behalf of Jan Kundrát.
Committed on 24/03/2019 at 21:49.
Pushed by gerrit into branch 'master'.

Merge changes from topic 'bug-404211'

* changes:
  Settings: SMTP: wrap the warning to avoid a horizontal scrollbar
  Settings: IMAP: remove placeholder string
  Improve port number warnings
  GUI: Settings: simplify stylesheet manipulation


https://commits.kde.org/trojita/681c99cf2429137f4462bf61232c4bb8a5a363cf