Add version() to each plugin

And display it in the screensaver tool with the -l option.

modules/Huey/Huey.cpp

@@ -12,6 +12,7 @@ class Huey : public ScreensaverPlugin {
         int fps() const override;
         void update() override;
         void render() override;
+        std::string version() const override;
 
     private:
         double _hue {0};
@@ -21,6 +22,10 @@ ScreensaverPlugin* create_instance() {
     return new Huey;
 }
 
+std::string Huey::version() const {
+    return "1.1.0";
+}
+
 int Huey::fps() const {
     return 50;
 }