Save spent time on activities

Use set_active and save events to keep track of the time,
in seconds, that the activity is on the main screen. The
time is calculated and stored each time an activity is
open/resumed and closed. Therefore, each activity entry
will have an array of seconds for each time it was used.

This makes a 1 to 1 relationship between the spent and
lauch times. In orders words, we can reflect not only the
"when", but also the " for how long".

Even though this time does not necessarily represents the
real time the user "used" the activity, it is a low-cost
approach for reflecting time.

This is a long requested featured from deployments that have
been using statistics tools on journal metadata, and this
patch is already being used by OLPC-AU.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Merge branch 'tchx84-spent-times-try1'