[ADD] project: add my followed task filter

There is currently no 'followed task' filter. Making custom filter does
not work as it erases existing search. When working in some projet context
you do not want to loose your existing search criterions. We therefore
choose to add a new 'my followed task' filter.

We could have made ir.filters not exclusive aka add them to existing
search criterions but "some people" did not want.

Last but not least, we add this filter because FGI requested it.
It seems sufficien to me.

addons/project/views/project_views.xml

@@ -28,6 +28,7 @@
                     <field name="user_id"/>
                     <field name="stage_id"/>
                     <filter string="My Tasks" name="my_tasks" domain="[('user_id','=',uid)]"/>
+                    <filter string="My Followed Tasks" domain="[('message_is_follower', '=', True)]" />
                     <filter string="Unassigned" name="unassigned" domain="[('user_id', '=', False)]"/>
                     <separator/>
                     <filter string="Starred" name="starred" domain="[('priority','=',1)]"/>