merged video and audio nodes into a single media node

They performed exactly the same function so I don't think we need the clutter.

Also bumped the project version, but old projects are automatically converted.

app/core.cpp

@@ -71,7 +71,7 @@
 namespace olive {
 
 Core* Core::instance_ = nullptr;
-const uint Core::kProjectVersion = 201003;
+const uint Core::kProjectVersion = 201118;
 
 Core::Core(const CoreParams& params) :
   main_window_(nullptr),