Set limit to open activities and open instances

At the request of OLPC AU (in an effort to reduce OOM freezes) this
patch uses gconf to set a maximum number of open activities [1]. An
alert is shown if the user tries to launch more activities than the
maximum asking them to close an activity before opening a new one. If
maximum_number_of_open_activites is not set or == 0, then there is no
maximum limit applied.

Further, Some activities don't behave well if more than one instance
is open (e.g., SL #4554). This patch sets a limit on the number open
instances of an activity based on a new field in activity.info:
one_instance.

If and only if the maximum_instances field is present in
activity.info, is it used to set the limit of open instances.

NOTE: there is a patch to activitybundle.py in the toolkit necessary
for this patch to be used.

[1] /desktop/sugar/maximum_number_of_open_activities