Core: Make member functions const/static

readability-make-member-function-const finds lots of member functions
that could be made const. This change just picks getter functions that
really should be const.

readability-convert-member-functions-to-static finds non-static member
functions which do not access this. This change turns most of them
into static ones, but leaves some non static to keep the class API
consistent.

Change-Id: I004d6f7fab4375f9902f940f29b4a4f561fc7604
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/coreplugin.cpp

@@ -270,7 +270,7 @@ QObject *CorePlugin::remoteCommand(const QStringList & /* options */,
         });
         return nullptr;
     }
-    IDocument *res = m_mainWindow->openFiles(
+    IDocument *res = MainWindow::openFiles(
                 args, ICore::OpenFilesFlags(ICore::SwitchMode | ICore::CanContainLineAndColumnNumbers | ICore::SwitchSplitIfAlreadyVisible),
                 workingDirectory);
     m_mainWindow->raiseWindow();