Add "Delete" actions to site tree right click menu and page action buttons

[aoneil]

From http://www.silverstripe.com/google-summer-of-code-forum/flat/1636?showPost=1859#post1859 "There are three places to perform page actions a) on the page itself, were you have Save/Publish, etc. It currently has "Delete" missing. I suggest Delete is added. This provides the buttons used most of the time, especially for novice users, so put all these functions together.

b) in the right-click context menu of the site tree. By adding Delete, Save and Publish we provides a speed up for advanced users, who now don't need to navigate to a page to delete or publish it, saving time and making the product more enjoyable to use."

Moved from GSOC #72.