Let cross-editor plugins be button-less (aka 'extensions')

Cross editor plugins are not able to define the keys 'extensions' or 'buttons' in their hook implementations - like implementations of the native plugin hook can - they are assumed to need one button to function.

A good example of where this is not needed is the WYSIWYG Linebreaks module.

This request originated from #1045730: Hide the toolbar button.

For reference:
The cross-editor hooks are hook_wysiwyg_include_directory() and hook_INCLUDE_plugin() and the native plugin hook is hook_wysiwyg_plugin().
These hooks are documented in wysiwyg.api.php and changes to them would be considered an API change.
Though, it might be possible to provide backwards compatibility by first checking for 'buttons' or 'extensions', and then falling back to the regular 'icon path/file/title' keys.