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/mainwindow.cpp

@@ -148,7 +148,7 @@ MainWindow::MainWindow()
 
     QApplication::setStyle(new ManhattanStyle(baseName));
     m_generalSettings->setShowShortcutsInContextMenu(
-        m_generalSettings->showShortcutsInContextMenu());
+        GeneralSettings::showShortcutsInContextMenu());
 
     setDockNestingEnabled(true);