CppTools: Turn some classes into pure value types

ProjectInfo, ProjectPart and ProjectUpdateInfo used to carry pointers
to Project and/or Toolchain, even though they were used in contexts
where these pointers were either unsafe to access or not guaranteed to
be valid anymore, which made their use difficult and error-prone.
We turn these classes into pure value types by copying in all relevant
information before the first async operation takes place.

Fixes: QTCREATORBUG-25678
Change-Id: I1914b0dbda6c7dfba6c95e5e92f2d69977755590
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

tests/auto/cplusplus/cplusplusautotest.qbs

@@ -2,6 +2,7 @@ import qbs
 
 QtcAutotest {
     Depends { name: "CppTools" }
+    Depends { name: "ProjectExplorer" }
     Depends { name: "TextEditor" }
     Depends { name: "Utils" }
     Depends { name: "Qt.widgets" } // For QTextDocument & friends