Introduce Project::projectFilePath()

Use it instead of retrieving this information from the document.

Change-Id: I809fcb2daf59021cf503c371a5d40d75d7448796
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>

src/plugins/projectexplorer/project.cpp

@@ -109,6 +109,11 @@ Project::~Project()
     delete d;
 }
 
+QString Project::projectFilePath() const
+{
+    return document()->filePath();
+}
+
 bool Project::hasActiveBuildSettings() const
 {
     return activeTarget() && IBuildConfigurationFactory::find(activeTarget());