Implement plugin sorting functionality

Add support for sorting plugins by status, name,
or source in both ascending and descending order.

src/slic3r/GUI/PluginSource.hpp

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <string>
+
+namespace Slic3r
+{
+    namespace GUI
+    {
+        enum class PluginSource
+        {
+            // IMPORTANT: ordinal order is the Plugins dialog Source sort priority.
+            Mine,
+            Subscribed,
+            Local
+        };
+
+        inline std::string to_string(PluginSource source)
+        {
+            switch (source)
+            {
+            case PluginSource::Mine: return "mine";
+            case PluginSource::Subscribed: return "subscribed";
+            case PluginSource::Local: return "local";
+            }
+
+            return "local";
+        }
+    }
+} // namespace Slic3r::GUI