Introduce QML to Quassel

Introduce QML to Quassel

This marks the beginnings of QML widgets in Quassel. The first widget to be ported is ChatView;
this commit does the following:

 * Extend the build system for supporting QML
 * Add a new qmlui module
 * Add a (very) rudimentary ChatView component that connects to
 * the new QmlMessageModel, which is intended for proxying between the client and the QuickUI

For now, -DWITH_QML=ON is required for cmake to build Quassel with QML. No, it's not worth trying out
at the moment.