WIP to G-code export parallelization through pipelining:
Decoupled CoolingBuffer from GCode / GCodeWriter, ready to be
pipelined on a different thread.
Refactoring of GCode export of color changes, extruder switches etc,
so that the "color change" like extruder switches are applied first
at the Wipe Tower / G-code export, so that adding / removing
an extruder switch at the G-code preview slider does not invalidate
slicing.