Use COMPONENTS to find the Qt5 parts we need.

This goes through Qt5Config.cmake, which defines the minimum CMake
requirement for Qt5, something we bypassed so far. Our minimum is lower
than the Qt5 one, so you get compilation/automoc issues without that check.
Complete the move to target-based include directory handling.
Fix finding stuff in bin/ from executables in libexec/.

The launcher was accidentally relying on gammaray being in PATH for this
to work.