CppTools: Move ProjectPart in its own header file

Also extracting inline HeaderPath class and change projects list in vector
because the size is  larger than a pointer.

Change-Id: I885fdff3fe9bccc877634d1615249755f5b674fd
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/plugins/cpptools/cpptools.qbs

@@ -86,6 +86,8 @@ QtcPlugin {
         "includeutils.cpp", "includeutils.h",
         "indexitem.cpp", "indexitem.h",
         "insertionpointlocator.cpp", "insertionpointlocator.h",
+        "projectpart.cpp", "projectpart.h",
+        "projectpartheaderpath.h",
         "searchsymbols.cpp", "searchsymbols.h",
         "semantichighlighter.cpp", "semantichighlighter.h",
         "senddocumenttracker.cpp", "senddocumenttracker.h",