Add version() to each plugin

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

src/main.cpp

@@ -41,8 +41,9 @@ void print_help() {
 }
 
 void list_plugins() {
-    for (const std::string& name : sc::plugin<ScreensaverPlugin>::names()) {
-        std::cout << name << '\n';
+    for (const auto& elem : sc::plugin<ScreensaverPlugin>::all()) {
+
+        std::cout << elem.first << " (" << elem.second()->version() << ")\n";
     }
 }