Add callback for more fine grained status updates

[hholzgra]

See also https://github.com/hholzgra/maposmatic/issues/93

Add a customizable callback for providing progress status information to a calling application every once in a while.

• [x] add general infrastructure
• [x] add status update messages to single page renderers
• [x] add status update messages to multi page renderer
• [x] i10n / l18n