Remove the old API assembly invalidation system. It's pretty simple since now the editor has a hard dependency on the API assemblies and SCons takes care of prebuilding them.
If we fail to load a project's API assembly because it was either missing or outdated, we just copy the prebuilt assemblies to the project and try again. We also do this when creating the solution and before building, just in case the user removed them from the disk after they were loaded.
This way the API assemblies will be always loaded successfully. If they are not, it's a bug.
Also fixed:
- EditorDef was behaving like GlobalDef in GodotTools.
- NullReferenceException because we can't serialize System.WeakReference yet. Use Godot.WeakRef in the mean time.
Make the build system automatically build the C# Api assemblies to be shipped with the editor.
Make the editor, editor player and debug export templates use Api assemblies built with debug symbols.
Always run MSBuild to build the editor tools and Api assemblies when building Godot.
Several bugs fixed related to assembly hot reloading and restoring state.
Fix StringExtensions internal calls not being registered correctly, resulting in MissingMethodException.
