Ensure legacy display is destroyed before display.

SDL was being shut down before the legacy display could execute its
constructor, so SDL_DestroyTexture() was failing.
Add suppression for std::string.

Valgrind's memcheck was triggering on a lot of strings.
Since I'm not seeing the same entries on another system, I'm assuming
that they can be safely suppressed.
Add suppression for _XimLocalCreateIC.
Disable pool allocator in libstdc++ for Valgrind.
Disconnect displayConfigChanged signal.