* src/misc/variables.c, ALL: improvements to the object variables api.

* src/misc/variables.c, ALL: improvements to the object variables api.
   - added another argument to var_Change().
   - added a VLC_VAR_SETTEXT and VLC_VAR_GETTEXT method to var_Change()
     that are used to set/get a friendly name to an object variable.
   - added VLC_VAR_CHOICESCOUNT to var_Change() to be able to retrieve
     the number of availabel choices without having to get their value
     as well.
   - VLC_VAR_ADDCHOICE uses the extra var_Change() argument to assign
     a friendly name to the choice value.
   - VLC_VAR_GETCHOICES retrieves the choices values and their friendly
     names.

* modules/gui/wxwindows/*: first step in implementing menu auto-generation in the wxWindows interface.