add convenience function "notify_has_server_cap" to notify.h

[Alexander Sack]

Inspired by http://bugzilla.gnome.org/show_bug.cgi?id=584857#c2 I added a convenience function to libnotify that allows easy testing for certain capabilities: gboolean notify_has_server_cap(const char* capability)

Attaching patch.

[Alexander Sack (asac)]

PPA builds failed due to coreutils bustage; attaching debdiff so one can test.

[Alexander Sack (asac)]

forwarded upstream: http://trac.galago-project.org/ticket/188

[A. Walton (awalton)]

g_strcmp0 bumps the GLib requirement for libnotify by quite a bit and the comments are in what looks to be doxygen format when they should be in gtk-doc format.

Otherwise it's fine by me.

[Alexander Sack (asac)]

i think doc format is in line with currently used format in notify.* ... we should do that in another bug.