Support for global plugins which can globally bind scripts, handle NVDAObject events and choose NVDAObject overlay classes. See globalPluginHandler.GlobalPlugin for details.
Note that there is no longer an _default app module. Global scripts should be added or overridden using global plugins.
App modules should instead subclass appModuleHandler.AppModule (the base AppModule class).
The scripts from the _default app module are now in the new globalCommands.GlobalCommands object.
Fixes #281.