elementary: Move icon lookup order to user specified theme.

As discussed on the mailing list we're taming the icon_standard code.
This means that icon themes are set by the user not the app.
Deprecate all the icon_lookup calls etc and remove the behaviour.
Add a config panel to elementary_config where the theme can be selected.

diff --cc src/lib/elementary/elm_toolbar.c
Simple merge

diff --cc src/lib/elementary/elm_widget_toolbar.h
Simple merge