Perform spellchecker setup on creation

When spellchecking was added to WebEnginePart, SpellCheckerManager was a singleton class, which meant we couldn't pass a QWebEngineProfile to its constructor and had to do all initialization in a setup method. With the introduction of WebEnginePartControls, the profile is passed to SpellCheckerManager constructor, so there's no need to have the setup method anymore.

This also fixes a bug introduced when SpellCheckerManager stopped being a singleton, since its setup method wasn't called anymore, causing the user preferences not being loaded unless he manually opened the spell checker configuration dialog.

This fixes bug 434173. Unfortunately, I forgot to mention this in the commit message. Is it possible to change it before merging?