- svg is the page elements from the svg.
- num_menu_buttons is the Number of menu buttons; starting from the bottom left then right, then top, then left.
- button prefix (e.g MI_) is the prefix of the labels in the SVG for the menu boxes.
- _canvas is the canvas group.
- designation (optional) is used for Emesary designation
NOTE:
This does not actually create the canvas elements, or parse the SVG, that would typically be done in
a higher level class that contains an instance of this class.
see: http://wiki.flightgear.org/Canvas_MFD_Framework