Merging the two projects into one with build configurations

Removed the "src_2018" project, since it's now 1:1 with the main project and the only difference is references. Now using build configurations and preprocessor directives to accomplish the same thing.

src/MainMenu/MainMenu.cs

@@ -56,7 +56,7 @@ namespace Explorer
                 var origSkin = GUI.skin;
                 GUI.skin = UIStyles.WindowSkin;
 
-                MainRect = GUI.Window(MainWindowID, MainRect, (GUI.WindowFunction)MainWindow, "IL2CPP Runtime Explorer");
+                MainRect = GUI.Window(MainWindowID, MainRect, (GUI.WindowFunction)MainWindow, CppExplorer.NAME);
 
                 GUI.skin = origSkin;
             }