Move watch_later to $XDG_STATE_HOME

[etircopyh]

Not too long ago a new basedir specs were presented and brought us XDG_STATE_HOME, for storing, well, applications state stuff (e.g history, saved layouts, etc).

Seeing as watch_later directory indeed contains aforementioned content it seems logical to follow on specs suggestions.

Expected behavior of the wanted feature

Place watch_later to XDG_STATE_HOME (~/.local/state if variable's not set) by default. Perhaps full path being ~/.local/state/mpv/watch_later would be better than just dropping watch_later there, so user can distinguish to which app it belongs.

[Dudemanguy]

Personally not opposed to this. The biggest issue with changing the default directories is that it also affects macos (since it utilizes the same code path) which is kind of icky.