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/slic3r/Utils/PrintHost.hpp

@@ -10,6 +10,7 @@
 
 #include "Http.hpp"
 
+class wxArrayString;
 
 namespace Slic3r {
 
@@ -20,6 +21,9 @@ struct PrintHostUpload
 {
     boost::filesystem::path source_path;
     boost::filesystem::path upload_path;
+    
+    std::string group;
+    
     bool start_print = false;
 };
 
@@ -41,8 +45,15 @@ public:
     virtual bool has_auto_discovery() const = 0;
     virtual bool can_test() const = 0;
     virtual bool can_start_print() const = 0;
+    // A print host usually does not support multiple printers, with the exception of Repetier server.
+    virtual bool supports_multiple_printers() const { return false; }
     virtual std::string get_host() const = 0;
 
+    // Support for Repetier server multiple groups & printers. Not supported by other print hosts.
+    // Returns false if not supported. May throw HostNetworkError.
+    virtual bool get_groups(wxArrayString & /* groups */) const { return false; }
+    virtual bool get_printers(wxArrayString & /* printers */) const { return false; }
+
     static PrintHost* get_print_host(DynamicPrintConfig *config);
 
 protected: