Aggregation/Utils/ExtensionSystem: 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.

readability-static-accessed-through-instance fixes the places where
the originally non-static, now static functions were called through
instance.

Change-Id: I8cf16c01f7988a7c9d073b5f8ede6a9706b94fb0
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/icore.cpp

@@ -790,7 +790,7 @@ void ICore::addPreCloseListener(const std::function<bool ()> &listener)
 */
 QString ICore::systemInformation()
 {
-    QString result = PluginManager::instance()->systemInformation() + '\n';
+    QString result = PluginManager::systemInformation() + '\n';
     result += versionString() + '\n';
     result += buildCompatibilityString() + '\n';
 #ifdef IDE_REVISION