D10388: allow to have more than one default activity

[Marco Martin]

mart added reviewers: ivan, bshah.

REPOSITORY
  R161 KActivity Manager Service

REVISION DETAIL
  https://phabricator.kde.org/D10388

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