applet: return icon name from get_icon() whenever possible (bgo #740574)

Get rid of all unnecessary pixbuf passing and prepare for appindicator
support that requires icon name passing instead.

https://bugzilla.gnome.org/show_bug.cgi?id=740574
applet: small fixups for "return icon name from get_icon() whenever possible" (bgo #740574)

https://bugzilla.gnome.org/show_bug.cgi?id=740574
add applet_schedule_update_menu

Add an empty applet_schedule_update_menu() that would later be used
by the appindicator code.
prepare for appindicator patches

A few trivial changes to make it easy to support appindicator later.
indicator: add support for --enable-indicator

Add new configure option --enable-indicator which implies a optional
dependency on libappindicator. This is the way systray is done in the
current versions of Unity, KDE and Enlightenment and others. Some of the
systray implementations already dropped support for Xembed, others are
going to, mostly to gain independence on X11 as part of being ported to
Wayland.

Keep the previous functionality intact.
indicator: add device specific icon code

Add device specific code for appindicator.
trivial: use assert, not g_return_if_fail