* ALL: a few updates to the variables API:
  - got rid of the VLC_VAR_COMMAND variable type and introduced the VLC_VAR_ISCOMMAND flag
instead. This simplifies things and avoid the strange command variables behaviour. Command
variables will now trigger their callback on var_Set() like any other variables (look in
modules/control/rc/rc.c for an example).
  - Renamed the VLC_VAR_ISLIST flag into VLC_VAR_HASCHOICE.
  - Added a new VLC_VAR_VOID variable type. This can be useful in conjunction with
VLC_VAR_ISCOMMAND for instance.