Replace source file viewer with one that actually works.

This is much simpler, custom implementation of a text file viewer. It is
able to perform these two tasks as intended:
- center source view on the selected line,
- highlight that line.

manual/tracy.tex

@@ -2741,7 +2741,6 @@ The following libraries are included with and used by the Tracy Profiler. Entrie
 \item MIT license
 \begin{itemize}
 \item Dear ImGui -- \url{https://github.com/ocornut/imgui}
-\item ImGuiColorTextEdit -- \url{https://github.com/BalazsJako/ImGuiColorTextEdit}
 \item JSON for Modern C++ -- \url{https://github.com/nlohmann/json}
 \item robin-hood-hashing -- \url{https://github.com/martinus/robin-hood-hashing}
 \end{itemize}