gschem: Add scrollbar support to GschemPageView
gschem: Rename klasse -> klass (in keeping with all other GObject code)

This is for consistency with other GObject code in gEDA, and matches the
idioms in GTK and GLib. By convention, "class" is only misspelled "klass"
when used alone (to avoid clashing with the C++ reserved word).

Variables "object_class", "parent_class" etc.. are not usually misspelled.
gschem: Clean code introduced in 6a12031ed00900a472949c0cdf0d1eddfe9056c2

Fixup some broken whitespace, re-locate the function to avoid requiring a
forward declaration and rejig the parameters (to take coordinates) so we
can use a more descriptive name.
Only include x_multiattrib.h from x_multiattrib.c

This speeds up builds whilst I'm changing that header, and hides private
implementation details of the classes within.

The exported functions from x_multiattrib.c are defined in prototype.h,
so only x_multiattrib.c needs the header.
x_multiattrib.[ch]: Make multiattrib_update() static to x_multiattrib.c
Fix zooming (broken in commit fb958cc4f6aec2b2a3b13a72f78d3dff2ae6ada9)

Attach our x_event_scroll() handler to the drawing area, not the main
window, so we receive the event before the GtkScrolledWindow container.

Futhermore, x_event_scroll() now returns TRUE, which stops further
processing of the event after our handler. This avoids the built-in
GtkScrolledWindow signal handler also trying to act on the events.