Debugging UI extension headers via Dear ImGui (#125)

This PR adds optional debugging UI extension headers (or rather: currently only one for
sokol_gfx.h called sokol_gfx_imgui.h). sokol_gfx.h gets a new API function to install callback
functions for tracing the public sokol_gfx.h API calls.

README.md

@@ -407,6 +407,20 @@ Mainly some "missing features" for desktop apps:
 
 # Updates
 
+- **05-Mar-2019**: sokol_gfx.h now has a 'trace hook' API, and I have started
+implementing optional debug-inspection-UI headers on top of Dear ImGui:
+    - sokol_gfx.h has a new function sg_install_trace_hook(), this allows
+      you to install a callback function for each public sokol_gfx.h function
+      (and a couple or error callbacks). For more details, search for "TRACE HOOKS"
+      in sokol_gfx.h
+    - I have started a new 'subproject' in the 'imgui' directory, this will
+      contain a slowly growing set of optional debug-inspection-UI headers
+      which allow to peek under the hood of the Sokol headers. The UIs are
+      implemented with [Dear ImGui](https://github.com/ocornut/imgui). Again,
+      see the README in the 'imgui' directory and the headers in there 
+      for details, and check out the live demos on the [Sokol Sample Webpage](https://floooh.github.io/sokol-html5/)
+      (click on the little UI buttons in the top right corner of each thumbnail)
+
 - **21-Feb-2019**: sokol_app.h and sokol_audio.h now have an alternative
 set of callbacks with user_data arguments. This is useful if you don't
 want or cannot store your own application state in global variables.

fips.yml

@@ -1,2 +1,2 @@
 exports:
-  header-dirs: [ . ]
+  header-dirs: [ ".", "imgui" ]