Don't call ConfigDialog::applyChanges() if nothing has changed

The OK button is always enabled, so it can be clicked when nothing is modified on the current page. Calling page->apply() and emitting configSaved() is useless and wasteful in this case.

One downside of this optimization is that if a change of some widget on some ConfigPage is not connected to that page's changed() signal, clicking OK button no longer saves that setting, unless another "connected" widget on that page is also changed and the Apply button is enabled. However such missing connection is problematic in other ways: the Apply button does not become enabled; switching to a different ConfigPage silently discards a lone "unconnected" change. Therefore such a bug can be fixed properly only by adding the missing connection.

!301 (merged) adds two missing connections. Hopefully all other widgets are "connected". If not, this optimization could reveal a bug, which would then be fixed.