Fixed a bug: Wrong save of a preset if the preset is already exist.

Step to repro:
1. Select system "Some_preset" preset
2. Make some changes
3. Save preset with "Some_preset - Copy" name.
=>  "Some_preset - Copy" is selected, there is/are marked option(s) only different from system preset.
repeate 1-3 steps
=>  "Some_preset - Copy" is selected, there is/are marked option(s) different from DEFAULT preset.

src/slic3r/GUI/Preset.hpp

@@ -276,6 +276,9 @@ public:
     // Delete the current preset, activate the first visible preset.
     // returns true if the preset was deleted successfully.
     bool            delete_current_preset();
+    // Delete the current preset, activate the first visible preset.
+    // returns true if the preset was deleted successfully.
+    bool            delete_preset(const std::string& name);
 
     // Load default bitmap to be placed at the wxBitmapComboBox of a MainFrame.
     void            load_bitmap_default(wxWindow *window, const std::string &file_name);