intel: tools: import ImGui

intel: tools: import ImGui

We want to add a new UI tool to decode aub files. This will use the
Dear ImGui library to render its interface. The build of this UI
toolkit is conditional to -Dwith_tools=intel-ui which superseeds
-Dwith_tools=intel.

The main way to use ImGui is to embed its source code at a particular
revision. Most embedding projects have to do a bit of integration
which is really specific to one's project. In our case the only
modification is to include libepoxy. We also choose to use Gtk+3 for
the window system integration. As oppose to the previous previous
version of this patch using GLFW, Gtk+ is able to handle X11/Wayland
session as well as property DPI scaling on retina monitors.

The import was done at this commit (https://github.com/ocornut/imgui) :

commit 6211f40f3d903dd9df961256e044029c49793aa3
Author: omar <omarcornut@gmail.com>
Date:   Fri Jul 27 12:29:33 2018 +0200

    Internals: Drag and Drop: default drop preview use a narrower clipping rectangle (no effect here, but other branches uses a narrow clipping rectangle that was too small so this is a fix for it) + Comments

v2: Switch from GLFW to GTK+ (Lionel)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Rafael Antognolli <rafael.antognolli@intel.com>