Fix NotificationPulsingIcon property name

Use file instead of icon_filename. The later does
not exists.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Display badges in activities tray icons

Modify ActivityButton so it uses NotificationPulsingIcon and
complies with the show_badge and hide_badge convention.

Add and remove activity buttons from by_button index.
Add NotificationBox to current activity palette

Include a NotificationBox while creating palette in
CurrentActivityPalette class.

This will display notifications received from the
activity. The activity is identified by home_activity
id.