Move Interface Builder document handling to its own module.

New/changed functionality:
- Properly sets minimum OS deployment target.
- Works if sysroot is not set.
- Provide some options for the user to set.
- Custom flags to ibtool compiler.
- Support both xib and nib extensions.

Task-number: QBS-328

Change-Id: I8702e83ddc31b00eef8ff72fb1b3fb8341babcc9
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>