- solidify/level.c (expose_event, level): renamed expose_event to
expose_event_xy, for consistency with new expose event handlers
- solidify/level.c (expose_event_xz, expose_event_zy, level): added expose
event handlers for the auxiliary projections
- solidify/level.c (level): the drawing areas receive an expose event when
becoming visible, so there's no need to explicitly initilize them
- solidify/level.c (xz, draw_xz, motion_notify_event, level): show a
projection on a xz plane at the cursor position
- solidify/level.c (zy, draw_zy, motion_notify_event, level): show a
projection on a zy plane at the cursor position
