Assign max_participants in activity.info

As part of an effort to "honor" max_participants, this patch supports setting
max_participants in activity.info, thus making it available in the bundle.

By default, if it is not set in the bundle, the previous behavior persists.

In support of this change, a cache of Activity bundles is
maintained. The goal is to eliminate unnecessary calls to the file
system.
Release 0.101.5
Pep8 fix
Add a method to get all the extensions associated to a mime type

This is needed to solve problems with mismanaged extensions in the journal.
Test commit
Test commit
Pep8 and pyflakes fixes to sugar-activity
Avoid breaking activity startup by dbus timeout on sigle instance - Fixes #4773

This problem only happen in activities configured with the option -s
in the activity.info "exec" field, and the objective is use a single process,
even when starting more than one instance. If the process do not reply,
start a new process, instead of show a error.

Signed-off-by: Gonzalo Odiard <godiard@sugarlabs.org>