[imgui] Allow usage of imgui_stdlib (#2889)

This bumps the version number of thirdparty-imgui in Gradle and adds
imgui_stdlib.cpp into the sources in CMake, as well as adding a new
include directory.

shared/config.gradle

@@ -11,7 +11,7 @@ nativeUtils {
       niLibVersion = "2020.10.1"
       opencvVersion = "3.4.7-5"
       googleTestVersion = "1.9.0-5-437e100-1"
-      imguiVersion = "1.76-9"
+      imguiVersion = "1.76-10"
       wpimathVersion = "-1"
     }
   }