Add version() to each plugin

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

modules/Whirling/Whirling.cpp

@@ -35,6 +35,7 @@ class Whirling : public ScreensaverPlugin {
         void update() override;
         void render() override;
         void configure() override;
+        std::string version() const override;
 
     private:
         std::vector<Lissajous> knots;
@@ -46,6 +47,10 @@ ScreensaverPlugin* create_instance() {
     return new Whirling;
 }
 
+std::string Whirling::version() const {
+    return "1.0.0";
+}
+
 int Whirling::fps() const {
     return 30;
 }