nitpicker: add 'session_control' RPC function

The new 'session_control' function can be used to perform operations on
the global view stack that span one or multiple sessions, e.g., bringing
all views of specific sessions to the front, or hiding them.
gems: nit_fader
gems: add include/gems/animator.h

The utility for animating GUI elements was formerly private to
'app/decorator'.
gems: move reusable wm headers to include/gems
gems: add gems/wrapped_nitpicker_session.h

The 'Wrapped_nitpicker_session' contains the boiler-plate code that is
needed when virtualizing the nitpicker session interface.
gems: new menu-view application

The menu view generates a simple dialog of widgets and reports the
hovered element. It is meant to be embedded into applications that
require simple GUIs but don't want to deal with the pecularities of
a full-blown widget set.
gems: launcher application