Qt5::Widgets is required when building the UI.

(cherry picked from commit 2e883bf5b664d6fb52af4d1901e44635fdc268a8)
Build this test only if Qt5::Widgets is available.

(cherry picked from commit 51e86a2eefd0fcce16ada9d0655447e58cef5ae6)
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.
GAMMARAY_BUILD_UI implies we have widgets.

So we can simplify these conditions to not check for widget availability
as well.