ProjectExplorer: Avoid copy for node path

It appears to take 7% of project loading...

Change-Id: I2d20d919cc861a6ee95af359a584fb27a915cf80
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>

src/plugins/projectexplorer/projectnodes.cpp

@@ -155,7 +155,7 @@ FolderNode *Node::parentFolderNode() const
 /*!
   The path of the file or folder in the filesystem the node represents.
   */
-Utils::FileName Node::path() const
+const Utils::FileName &Node::path() const
 {
     return m_path;
 }

src/plugins/projectexplorer/projectnodes.h

@@ -112,7 +112,7 @@ public:
     NodeType nodeType() const;
     ProjectNode *projectNode() const;     // managing project
     FolderNode *parentFolderNode() const; // parent folder or project
-    Utils::FileName path() const;                 // file system path
+    const Utils::FileName &path() const;  // file system path
     int line() const;
     virtual QString displayName() const;
     virtual QString tooltip() const;

src/plugins/resourceeditor/resourcenode.cpp

@@ -286,7 +286,7 @@ bool ResourceTopLevelNode::showInSimpleTree() const
 }
 
 ResourceFolderNode::ResourceFolderNode(const QString &prefix, const QString &lang, ResourceTopLevelNode *parent)
-    : ProjectExplorer::FolderNode(parent->path().appendPath(prefix)),
+    : ProjectExplorer::FolderNode(Utils::FileName(parent->path()).appendPath(prefix)),
       // TOOD Why add existing directory doesn't work
       m_topLevelNode(parent),
       m_prefix(prefix),