Add version() to each plugin

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

modules/Grid/Grid.cpp

@@ -35,6 +35,7 @@ class Grid : public ScreensaverPlugin {
         int fps() const override;
         void update() override;
         void render() override;
+        std::string version() const override;
 
     private:
         size_t _h;
@@ -109,6 +110,10 @@ void Grid::configure() {
     setup(_c, _r);
 }
 
+std::string Grid::version() const {
+    return "1.0.0";
+}
+
 int Grid::fps() const {
     return 30;
 }