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