update drracket/scribblings/drracket/interface-essentials.scrbl with new screenshot and change text from 'status line' to 'status area'.

ref #172 @rfindler

1. update drracket/scribblings/drracket/interface-essentials.scrbl with new screenshot and change text from 'status line' to 'status area'. perhaps use annotated screenshot from https://github.com/racket/drracket/pull/172#issuecomment-347038169

• new paragraph for 'signature boxes' with images of closed 'blue quarter circle', open, locked/unlocked, link to documentation.
  (currently 'The blue quarter circle in the upper right corner provides access to the summary information from the documentation.[...]' - rest of paragraph is 'status line')
• new paragraph for 'status area' with annotated image
  (currently '[...]The status line at the bottom of DrRacket’s window provides information about the current line and position of the editing caret, whether the current file can be modified, and whether DrRacket is currently evaluating any expression. The recycling icon flashes while DrRacket is “recycling” internal resources, such as memory.')

Open drracket/scribblings/drracket/interface-essentials.scrbl and compare the existing interface description with the annotated screenshot linked in the issue. Update the screenshot and wording for signature boxes and the status area, including the requested paragraphs and images; done means the file reflects those UI changes and the documentation builds successfully.