Redesign the topic widget

This replaces the old ugly hackish topicwidget by something cleaner. We now use a stacked widget to switch
between display and edit mode. In edit mode, we use a MultiLineEdit rather than a single QLineEdit.
Instead of doing voodooish resizing of the parent widget, we use proper sizeHints and sizePolicies that make
Qt's layout system actually work with this.

There are still some features missing though, notably clickable URLs and the clear button. Will
be implemented ASAP.

Closes #506, fixes #573, closes #608.