guiconfig: Add a graphical configuration interface

This is a graphical configuration interface written in Tkinter. Like
menuconfig.py, it supports showing all symbols (with invisible symbols
in red) and jumping directly to symbols. Symbol values can also be
changed directly from the jump-to dialog.

This interface should feel a lot smoother than menuconfig.py on Windows.

When single-menu mode is enabled, a single menu is shown at a time, like
in the terminal menuconfig. Only this mode distinguishes between symbols
defined with 'config' and symbols defined with 'menuconfig'.

Compatible with both Python 2 and Python 3. Has been tested on X11,
Windows, and macOS.

To avoid having to carry around a bunch of GIFs, the image data is
embedded in guiconfig.py. To use separate GIF files instead, change
_USE_EMBEDDED_IMAGES to False. The image files can be found in
https://github.com/ulfalizer/Kconfiglib/tree/screenshots/guiconfig

.

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