Spell checking

One of the most frequently requested feature for the Rekonq browser is support for spell checking.

Initial discussions with Morten indicate this feature is unlikely to go into Qt anytime soon because of the lack of such service on Windows. A solution might be to integrate spell checking as part of the platform plugins, so implementors can provide their own spell checking library.

An implementation for Linux and Mac should also be included to avoid duplicated effort for implementors.