Add support for scene/resource customization in export plugins

EditorExportPlugin adds a set of callbacks to allow customizing scenes, resources or subresources in all files exported:
* Can take scene files, resource files and subresources in all of them.
* Uses a cache for the converted files if nothing changes, so this work only happens if a file is modified.
* Uses hashing to differentiate export configuration caches.
* Removed the previous conversion code to binary, as this one uses existing stuff.

This API is useful in several scenarios:
* Needed by the "server" export platform to get rid of textures, meshes, audio, etc.
* Needed by text to binary converters.
* Needed by eventual optimizations such as shader precompiling on export, mesh merging and optimization, etc.

This is a draft, feedback is very welcome.

editor/export/editor_export.cpp

@@ -351,6 +351,8 @@ EditorExport::EditorExport() {
 
 	singleton = this;
 	set_process(true);
+
+	GLOBAL_DEF("editor/export/convert_text_resources_to_binary", true);
 }
 
 EditorExport::~EditorExport() {