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