Add version() to each plugin

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

modules/Default/Default.cpp

@@ -10,12 +10,17 @@ class Default : public ScreensaverPlugin {
         int fps() const override;
         void update() override;
         void render() override;
+        std::string version() const override;
 };
 
 ScreensaverPlugin* create_instance() {
     return new Default;
 }
 
+std::string Default::version() const {
+    return "1.0.0";
+}
+
 int Default::fps() const {
     return 2;
 }