doc: extend the documentation of DBus API

- add missing descrption for (Set|Delete)Element
- add more examples
- describe more problem elements

- related to #601

Signed-off-by: Jakub Filak <jfilak@redhat.com>
doc: install abrt-dbus documentation files to the correct places

- interface description to common dbus interfaces directory
- html documentation to shared doc directory

- closes #601

Signed-off-by: Jakub Filak <jfilak@redhat.com>