allow to have more than one default activity
Summary:
since addActivity in layout.js is the wrong place and extremely fragile,
allow to have more than one default activity with custom names
the config key is unchanged so all is retrocompatible, even
if the key being singular isn't 100% correct now
Test Plan: tried with an empty session, works correctly
Reviewers: Plasma, ivan, bshah
Reviewed By: Plasma, ivan
Subscribers: davidedmundson, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D10388