Make tinyMCE plugins definition configurable via widget properties

Currently, the plugins list if configured globally via an extension point system. We should keep this behavior for defining the default list.

Additionally, it should be possible to override the definition of the plugins via a new property on the TinyMCE widget. Since there can be several toolbars, we should use a convention like leveraging all the properties who start by "toolbar" (toolbar1, toolbar2)...