vulkaninfo: new vkconfig_output backdend

vulkaninfo: new vkconfig_output backdend

This commit adds a new backend, vkconfig_output, that prints the same information
as the text & html versions in a json format. This output will only be used in
vkconfig. Therefore it does not have a documented flag, but it is accesible with
'--vkconfig_output'.

Many parts of the code needed refactoring to handle the vkconfig_output output.
Replacing PrintElement with PrintString, requiring StartObject instead of
StartArray, and removing many branches to make the code simpler to read.

vkconfig_output does not nest json objects within json arrays, thus keeping every
non-leaf node of a json object named.

files modified
 - scripts/vulkaninfo_generator.py
 - vulkaninfo/generated/vulkaninfo.hpp
 - vulkaninfo/outputprinter.h
 - vulkaninfo/vulkaninfo.cpp
 - vulkaninfo/vulkaninfo.h

Change-Id: I33d036a75b65942db1ad62b2a1e0c341a2b5e36c