Profiler: Add a "Signposts" tab next to the "Samples" tab

This tab provides a filtered listing of all the signpost events in the
currently selected time range.

Userland/DevTools/Profiler/CMakeLists.txt

@@ -12,6 +12,7 @@ set(SOURCES
         Profile.cpp
         ProfileModel.cpp
         SamplesModel.cpp
+        SignpostsModel.cpp
         TimelineContainer.cpp
         TimelineHeader.cpp
         TimelineTrack.cpp