The PlaceholderParser is currently used by the GCode.cpp
and by Printer.cpp to generate a new name for the exported G-code or SVG file.
The PlaceholderParser::process() will throw a runtime_error with
a comprehensive error message.
The G-code export will include these error messages into the G-code text
with !!!!!! separators, and the GUI will inform the user, that the G-code
export failed.
https://github.com/prusa3d/Slic3r/issues/443
Change of the PlaceholderParser:
All vector configuration values stored into the PlaceholderParser
are expected to be addressed by the extruder ID, therefore
if a vector configuration value is addressed without an index,
a current extruder ID is used.
Also a small fix of fan handling: The fan speed is set to zero
at the start of the G-code if the cooling for the initial extruder
is disabled.
