Bug 26431: Use split button to offer choice of WYSIWYG or code editor for news

Bug 26431: Use split button to offer choice of WYSIWYG or code editor for news

This patch modifies the news interface so that the user is offered a
choice of editing news items using the default editor as defined by the
NewsToolEditor system preference or the non-default one.

To test, apply the patch and go to Tools -> News.

- In the list of news items you should see an "Edit" split button in
  each row.
- Clicking the "Edit" part of the button should open the edit view with
  the correct editor as defined by the NewsToolEditor preference.
- Clicking the menu arrow on the button should offer the other editing
  method:
 - If NewsToolEditor is set to "WYSIWYG," the menu should offer "text
   editor."
 - If NewsToolEditor is set to "text editor," the menu should offer
   "WYSIWYG editor."
- Test the button with the NewsToolEditor preference set to both options
  and confirm that the button and menu work correctly.

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>