QtTest is no longer mandatory.
Rely less on Qt4 compat code, but make it explicit what libraries we need.
Use CMAKE_INCLUDE_CURRENT_DIR.
Unset LD_PRELOAD much earlier.

This avoids propagating the probe to an immediately started QtWebProcess
for example.
Split web inspector plugin into probe and UI part.

No functional difference yet, but this will allow building a non-widget
WebKit2-only probe plugin eventually.