add new state syntax

[tsl0922]

Syntax:

#@state=<expr> is an improved version of #@prop:check to extend support for other state like disabled.

<expr> can be:

• static string like checked / disabled or checked,disabled
• Property Expansion that produces the above value (see expand-text)

#@prop:check is still suppored (but deprecated, and will be removed soon).

Usage

_           ignore                                 #menu: Disabled #@state=disabled
_           ignore                                 #menu: Checked #@state=checked

Ctrl+s      stop                                   #menu: Stop #@state=${?playlist-playing-pos==-1:disabled}
m           cycle mute                             #menu: Mute #@state=${?mute==yes:checked}

[zhongfly]

I prefer the expr syntax of Conditional auto profiles
You can use some codes in https://github.com/mpv-player/mpv/blob/master/player/lua/auto_profiles.lua

I don't think the syntax of Property Expansion is appropriate, especially when writing compound conditions, e.g. it's difficult to express "and" and "or".

[tsl0922]

#46