command: add display-names property

Add VOCTRL_GET_DISPLAY_NAMES and call it when the property is
requested. The vo should return the names of the displays that the mpv
window is covering.  For example, with x11 vos, xrandr names LVDS1,
HDMI1, etc.
vo/x11: implement VOCTRL_GET_DISPLAY_NAMES with xrandr names (e.g., "LVDS1")

XRRGetOutputInfo contains a "name" element which corresponds to to the
display names given to the user by the "xrandr" command line
utility. Copy it into the xrandr_display struct for each display.
On VOCTRL_GET_DISPLAY_NAMES, send a copy of the names
of the displays spanned by the mpv window on.