Add version() to each plugin

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

modules/FadingRects/FadingRects.cpp

@@ -40,6 +40,7 @@ class FadingRects : public ScreensaverPlugin {
         int fps() const override;
         void update () override;
         void render() override;
+        std::string version() const override;
 
     private:
         double _hue {0.0};
@@ -53,6 +54,10 @@ ScreensaverPlugin* create_instance() {
     return new FadingRects;
 }
 
+std::string FadingRects::version() const {
+    return "1.1.0";
+}
+
 int FadingRects::fps() const {
     return 20;
 }