feat: add secondary subtitle button

[coderHeJiyu]

Add a secondary subtitle track button next to the subtitle button.

The main use case is easier control of danmaku (bullet comments), which
are commonly provided as a secondary subtitle track.

The secondary subtitle button completely mirrors the existing subtitle
button:

• Left click: open the secondary subtitle track selector
• Right click: cycle secondary subtitle track
• Middle click / Shift+left click: cycle secondary subtitle track down
• Scroll wheel: cycle secondary subtitle tracks up/down

The icon temporarily reuses the existing subtitle icon.

Depends on mpv's built-in select-secondary-sid script binding and the
secondary-sid / current-tracks/sub2 properties.

Tested: yes