Merge of pull request Add support for RepetierServer #4384 by @docbobo

with the following refactorings:

1) Removed the "printhost_slug" config from the Printer config
   and from all the Printer config related spots.
2) "printhost_slug" renamed to "printhost_port". Slug sounds nasty.
3) Improved error reporting of RepetierHost class.
4) Refactored for the new "Physical Printers"

Following refactorings were done independently of the Repetier pull request:
1) Removed PrintHost static print config.
2) Clean-up after conversion of print host configuration
   from Printer config to Physical Printer config.
3) Fixed some issues, where the Printer config was still queried for
   host configuration. Vojtech believes that this should not happen
   after the host configuration is converted to physical printers.

Vojtech still feels that more refactoring is needed in regard to porting
the host configuration from Printer profile to the new Physical Printer
profile.

src/libslic3r/GCode.cpp

@@ -1619,7 +1619,7 @@ void GCode::_do_export(Print& print, FILE* file, ThumbnailsGeneratorCallback thu
     // Append full config.
     _write(file, "\n");
     {
-        std::string full_config = "";
+        std::string full_config;
         append_full_config(print, full_config);
         if (!full_config.empty())
             _write(file, full_config);
@@ -2475,6 +2475,7 @@ void GCode::append_full_config(const Print &print, std::string &str)
     static constexpr auto banned_keys = {
         "compatible_printers"sv,
         "compatible_prints"sv,
+        //FIXME The print host keys should not be exported to full_print_config anymore. The following keys may likely be removed.
         "print_host"sv,
         "printhost_apikey"sv,
         "printhost_cafile"sv