[DNT] Enable sending DoNotTrack header by default
[Cleanup] Use QZ_WS_X11 only for X11 specific code

In other cases, use Q_OS_UNIX for determining UNIX platform.
UNIX platfroms without X11 should benefit from this commit, including
MAC and UNIXes on Wayland.

Conflicts:
	src/lib/app/qupzilla.cpp
[Building] Don't require ending slash in USE_LIBPATH and QUPZILLA_PREFIX options