Use 'customContent' flag instead of 'bannerBody'

Use 'customContent' flag instead of 'bannerBody'

We used 'bannerBody' flag to differentiate the case when we move the banner to
the body when the notification is expanded from the one when we don't do that
and only use the custom content set for the notification, as is the case for
Telepathy notifications. We also always cleared the content of the notification
on update when bannerBody was set to true.

Flag named 'customContent' reflects the use case for it more clearly. The
comments that accompany it were also updated and improved.

We now always add the banner text as the first element in the expanded
notification unless 'customContent' flag is set to true.

If the 'body' parameter is specified, we use it in addition to the banner
text. The earlier version of the code had a bug that resulted in the 'body'
parameter not being set only in the case when the 'bannerBody' was set to
true and the banner text had newlines in it.

https://bugzilla.gnome.org/show_bug.cgi?id=623970