This is a very early version of the toggle. It sets subs to None when enabled, restores the previously selected tracks when disabled. Currently mapped as a default IINA command to the "Ctrl+v" shortcut and to the menu item "Disable Subtitles" under the Subtitles menu. It will update it's status automatically to On if the user manually sets both sub tracks to None. In that case, when the toggle is activated, it will set --sid=1 and --secondary-sid=0 by default. It will also automatically resets to Off whenever the user manually activates a subtitle track.

At the same time rename it from `disableSubtitles` to `toggleSubtitlesState`.

This commit also includes some non-functional changes automatically generated by Interface Builder.

Since `V` is mpv's subtitle visibility toggle, it's probably better to keep the key bindings as separate as possible. The plan is to have both visibility and state toggles in iina, and have them be user-configurable in preferences. The user can select which subtitle toggle features are enabled: neither, one, or both. The default setting is having the state toggle enabled and visibility toggle disabled.

Rename `subDisabled` to `subtitlesAreDisabled`, then change it to a computed property that equals to the negation of a new stored property `subtitlesAreEnabled`. The renaming brings it in line with Swift API guidelines. Favouring the positive version avoid double-negatives and make reasoning easier.

Consolidate sparsely located subtitles state-toggling functions into property observers for the subtitles state variable `subtitlesAreEnabled` and subtitles' track indices. This approach lowers coupling among different components.

This commit could've been separated into smaller commits, but Xcode has its limit in picking code snippet for staging.