Changed feed rate values and to be in mm/sec + more

Changed the feed rate buttons to cover a wider range and to show the
values in mm/sec instead of mm/min. Changed movement commands and quick
commands to use XY feed rate of 4000mm/min and Z feed r\ate of 100
mm/min. Display multiple extruder positions. Changed layout to allow for
increasing depth of extruder command table.
Added preliminary support for multiple heads

Added preliminary support for multiple extruders and a second hot end
heater
Minor changes for multiple heads