Track zoom changes

Deactivate activity when the zoom level is removed from activity.
Activate activity when the zoom level returns to the activity.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Refactor classes

Move Tracker logic to a proper class and leave
Account class as a empty launcher.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Rename service to tracker

Use tracker as the name of this service because it is more general,
considering that now we just "track" different kinds of events in
order to make activities usage-time calculations more precise.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Don't activate with the wrong zoom level

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Release 0.3.0-1

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>