vcl: move OutputDevice::ImplGetFrameDev() to Window

ImplGetFrameDev() is for some unknown reason in OutputDevice, but it's
most definitely something that's specific to Window. I've moved this, and
also renamed it getFrameDev() as it's a private function.

Change-Id: Iffb2c439bee8c29ee585a0d50cfd6fbcfc28c3bc
Reviewed-on: https://gerrit.libreoffice.org/12338
Reviewed-by: Chris Sherlock <chris.sherlock79@gmail.com>
Tested-by: Chris Sherlock <chris.sherlock79@gmail.com>