Error message when plugin overrides document class (happens anyway); document classes lose default after plugin deactivation

[zenogantner]

Currently (as of Padre 0.86), there is an error message if a plugin overrides a document class. The document class is overridden nonetheless.
When unloading a plugin, the default/initial document class is not restored.

How to reproduce:

1. In Padre::MimeTypes?, add a document class (e.g. Padre::Document::Config for Padre 0.86 or Padre::Document::PercentComment? for Padre 0.87/trunk) to the LaTeX MIME type.
2. Run ./dev
3. Load .tex file: comment toggling works
4. Load LaTeX plugin: there is the error message, plugin is loaded anyway
5. Unload LaTeX plugin
6. Load .tex file: no comment toggling is supported any more!

[zenogantner]

Fixed in r14769.

Here is what I did:

1. introduced variable %DEFAULT_DOC_CLASS in MimeTypes?.pm that stores the default classes
2. after initializing %MIME, the document classes are set from %DEFAULT_DOC_CLASS
3. add_mime_class(): removed the wrong error message
4. remove_mime_class(): renamed to reset_mime_class(), now reset to default document class (if one exists)