Gather editor related options in one box

Buttons related to user's preferences for configuration should be:

• grouped in the same box
• displayed in the configuration area, not in the main toolbar.

Current actions to gather:

• Switch between code & Wysiwyg editors
• Editors option (theme for code editor, Fullscreen mode later in NXS-4107)

These actions are not related to the save/discard/delete buttons.