[PATCH] headless: more work in vcl

[Riccardo Magliocchetti]

Hello,

provide HeadlessSalSystem::ShowNativeDialog() instead of having
it if-zeroed.
Then remove LIBO_HEADLESS ifdefery from 
unx/generic/plugadapt/salplug.cxx which we are not building anymore.

Please review

-- 
Riccardo Magliocchetti