Now that we have a wrapper exposable, we need to implement
on_mouse(). In it, we need to check if we have our own on-click
handler, or else, check if mouse is inside the sub-exposable, or in
the left- or right margin.
This is done by wrapping the selected particle/exposable in a
ramp-specific exposable. This way, we can add the margins before
delegating begin_expose() and expose() to the selected exposable.
This is intended to be a format-like string, with the possibility to
use tag formatters.
The expanded string will later be passed to the system() call.
