Ability to easily add phrases to the phrase book

Add a shortcut [done] and a context menu action to add the current translation unit to a phrase book.

Specifically out of scope are:

• Support drag and drop from the message pane to the phrase book window (which needs to become non-modal for that purpose). The above solution seems more usable.
• Option to add an entire translated file to the phrase book. This is the domain of translation memory (QTBUG-2574).