Fix the reloadOnChange setting

The setting allows the user to choose whether kgraphviewer should reload an open file when its content is changed on disk (the changes are tracked by a KDirWatch instance). So far, while the setting value chosen in the settings dialog was saved, it was ignored by the relevant program mechanisms, so every time a file was changed, the user would be asked if they want to reload it.

The setting was implemented in the main program class, KGraphViewer, but the reload logic is handled by DotGraphView, which in turn is a part of KGraphViewerPart. The proposed implementation adds a new KGraphViewer signal, reloadOnChangeModeChanged(), and connects it, through KGraphViewerPart, to a new DotGraphView method, setReloadOnChangeMode(); thus every time the setting is changed, it is propagated to the interested parties. The setting is also read every time a new file is open. While the existing textual setting values ("ask", "true", "false") are still used in the configuration file, the implementation introduces and uses another way of expressing them, the KGraphViewerInterface::ReloadOnChangeMode enum.

This patch conflicts, although hopefully in a minor way, with the pending Nicolas Fella’s Combine preferences into a single page (!3).