Adding K2BeforeSave plugin event

[styleware]

It's a plugin event used in previous version of K2.
It's very useful in some situations for thirdparty plugins.

[kavadas]

You can use the core events "onContentBeforeSave" and "onContentAfterSave". This events now supply the context argument so you can check if it is a save action for a K2 item, a K2 category, a Joomla! article etc. If there is something else that we are missing regarding this, let us know.

[styleware]

Yes, but that will require the creation of separate content plugin along with our K2 plugin just for the use of that event alone. It will be nice if the old method works also, so that the code can be in one place when possible.