Plug the core-apitrace support for performance counters to qapitrace

Apitrace is an open source program that allows tracing, replaying, inspecting and profiling OpenGL/Direct3D calls made by any application. Last summer during GSoC 2015 there was developed an abstract interface that allows to use various profiling backends in Apitrace, also several backends (that use OpenGL extensions) were implemented then. The goal of this task is to provide a convenient interface to the new feature in qapitrace. There already exists a version of profiling view in qapitrace. It has only a limited set of old metrics and it might lack some usability features. This GUI project plans to add new metrics, as well as improving the interface overall.