MPV: Adjust subtitle delay/offset (#470)

**This is for MPV playback backend only!** Since this only works in MPV
for now, the option is not shown if using ExoPlayer.

Adds a setting (the gear icon during playback) to adjust the subtitle
delay.

A negative delay shows subtitles sooner. A positive delay shows them
later. For example, if a subtitle is supposed to show between
1000ms-2000ms, a negative 250ms delay will show it at 750ms-1750ms
instead.

The delay is persisted between plays of the same media & subtitle track.

Ref: #12 


![subtitle_delay](https://github.com/user-attachments/assets/824e3a28-650a-4619-9535-b67addaabf1e)

app/src/main/java/com/github/damontecres/wholphin/util/mpv/MpvPlayer.kt

@@ -45,6 +45,7 @@ import com.github.damontecres.wholphin.util.mpv.MPVLib.MpvEvent.MPV_EVENT_PLAYBA
 import com.github.damontecres.wholphin.util.mpv.MPVLib.MpvEvent.MPV_EVENT_VIDEO_RECONFIG
 import timber.log.Timber
 import kotlin.concurrent.atomics.ExperimentalAtomicApi
+import kotlin.time.Duration
 import kotlin.time.Duration.Companion.seconds
 
 /**
@@ -799,7 +800,7 @@ class MpvPlayer(
         // no-op
     }
 
-    var subtitleDelay: Double
+    var subtitleDelaySeconds: Double
         get() {
             if (isReleased) return 0.0
             return MPVLib.getPropertyDouble("sub-delay") ?: 0.0
@@ -808,6 +809,16 @@ class MpvPlayer(
             if (isReleased) return
             MPVLib.setPropertyDouble("sub-delay", value)
         }
+
+    var subtitleDelay: Duration
+        get() {
+            if (isReleased) return Duration.ZERO
+            return subtitleDelaySeconds.seconds
+        }
+        set(value) {
+            if (isReleased) return
+            subtitleDelaySeconds = value.inWholeMilliseconds / 1000.0
+        }
 }
 
 fun MPVLib.setPropertyColor(