don't bother looking for KDSME unless we have Qt5
Add GAMMARAY_BUILD_UI option, alongside GAMMARAY_PROBE_ONLY_BUILD.

Their semantics were kinda mixed until now, but we need both:
- GAMMARAY_PROBE_ONLY_BUILD is for building another probe with a different
  ABI, while reusing an already built launcher and client.
- GAMMARAY_BUILD_UI is for disabling building the client and in-process UI
  plug-ins, for example when building for an embedded target.