Make "Internationlization with Qt" about the classes that
support i18n and move all other information to "Writing Source
Code for Translation".

Combine with information from the Qt Linguist Manual and remove
redundancy.

Change-Id: If7f4d7702ca7fcffcca262f33494867a028449d0
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>