Move the printer support backend into it's own plugin

Move the printer support backend into it's own plugin

QPlatformIntegration can't have a dependency onto printing
anymore now that printing lives in it's own library. Move
it into a plugin of it's own.

Change-Id: I3ec4b38f4336eb96d92ea799544d17af359c83e1
Reviewed-on: http://codereview.qt.nokia.com/3210
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>