input: don't override item and capabilities from the slave

This MR also adds input_CanPaceControl() that will be used from the player to detect if gapless can be enabled. cf. https://code.videolan.org/videolan/vlc/-/wikis/Meetingreports/2020-10-04-VLC-Tech-Meeting-via-videoconf#gapless

This MR depends on !59 (merged)