Core::Filemanager: make some methods static

This follows suit to the ICore changes.

Change-Id: Iba2de1b1e3f2574fd1459892eae702e6af1cc7dc
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>

src/plugins/tasklist/taskfilefactory.cpp

@@ -92,7 +92,7 @@ Core::IFile *TaskFileFactory::open(ProjectExplorer::Project *context, const QStr
     m_openFiles.append(file);
 
     // Register with filemanager:
-    Core::ICore::fileManager()->addFile(file);
+    Core::FileManager::addFile(file);
 
     return file;
 }