COMMON: Introduce SaveFileManager::exists

Checks if a savefile with given name exists.

Implement on all backends.

backends/platform/dc/vmsave.cpp

@@ -336,6 +336,10 @@ public:
 	}
 
 	Common::StringArray listSavefiles(const Common::String &pattern) override;
+
+	bool exists(const Common::String &filename) override {
+		return InVMSave().readSaveGame(filename.c_str());
+	}
 };
 
 void OutVMSave::finalize()