D8449: Have a default backend (if one available)

[Nathaniel Graham]

ngraham added reviewers: ivan, Plasma.

REPOSITORY
  R845 Plasma Vault

REVISION DETAIL
  https://phabricator.kde.org/D8449

To: cryptodude, ivan, #plasma
Cc: plasma-devel, ZrenBot, progwolff, lesliezhai, ali-mohamed, jensreuterberg, 
abetts, sebas, apol, mart