Add previous-/nextActivity methods

Summary:
These two methods can be used to switch to the previous/next activity in
alphabetical order. They are exposed via the DBus interface. This
functionality can also be accessed by kactivities(-cli) and plasmashell, which
currently both implement their own separate versions of it.

@ivan mentioned being a bit wary of keeping a sorted activity list in
kactivitymanagerd, so I've opted to only retrieve and sort the list on demand
here.

Reviewers: ivan

Reviewed By: ivan

Subscribers: ivan, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22381