WIP: Plugin Extensions are not detected with ClassLoadingStrategy.APD

[wolframhaussig]

solves #449

• Added Testcase

TODO: fix Bug

The possible solutions I currently see:

1. PluginClassLoader.getResource
   PluginClassLoader.getResource would have to be aware of the extensions.idx file and always use the plugin way to load the file. Downside: PluginClassLoader would have to be aware of the internals of the storage as adding a new filename would require to change the pluginClassLoader(e.g. adding a NewExtensionStorage which reads from META-INF/new.idx would require that META-INF/new.idx be added to the PluginClassLoader whitelist
2. LegacyExtensionFinder.readPluginsStorages
   LegacyExtensionFinder.readPluginsStorages would need to be changed from getResourceAsStream to getResources. This way, the extensionFinder would receive all possible URLs and if more than one url is returned it could decide which url to use(e.g. by comparing with plugin.getPluginPath() to get the correct url). This would partially duplicate the logic of the PluginClassLoader but would keep the classes independent.

@decebals What do you think?

[wolframhaussig]

I am not entirely happy with the solution yet but I want to get your opinion @decebals : Is this the right way to go?