doc: Add guiconfig documentation

Add documentation for the new GUI configuration interface in the
Application Development guide.

Also update all the images, touch up the language a bit, and tweak some
minor stuff:

 - Say *.conf instead of .conf to make it clearer that .conf isn't the
   literal name

 - Add a missing -D<board> argument when running cmake

 - Use figure:: instead of image::. Comes out left-aligned and with
   space between the images, which looks nicer.

 - Explain what '- -' and '-*-' means in the terminal menuconfig.

tkinter isn't included by default in many Python installations, despite
being part of the Python standard library, so also add the required
packages to the Development Environment Setup section of the manual. Add
a note to the Application Development section as well, to help
debugging.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>