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/cppcompletion_test.cpp

@@ -113,7 +113,7 @@ public:
             = new CppCompletionAssistInterface(m_editorWidget->textDocument()->filePath().toString(),
                                                m_textDocument, m_position,
                                                ExplicitlyInvoked, m_snapshot,
-                                               ProjectPart::HeaderPaths(),
+                                               ProjectPartHeaderPaths(),
                                                languageFeatures);
         InternalCppCompletionAssistProcessor processor;