Add setting for subtitle outline size (#375)

Adds a settings allowing for adjusting the thickness of unstyled
subtitle outlines, if outline is enabled.

This is applied for both ExoPlayer & MPV

Closes #179

![outline
Large](https://github.com/user-attachments/assets/dfef74a5-a4d4-49db-a8d2-231e46ab151e)

Dev note: the ExoPlayer implementation is bad code, don't write code
like this. It works though

app/src/main/java/com/github/damontecres/wholphin/preferences/AppPreferencesSerializer.kt

@@ -170,4 +170,5 @@ fun SubtitlePreferences.Builder.resetSubtitles() {
     backgroundOpacity = SubtitleSettings.BackgroundOpacity.defaultValue.toInt()
     backgroundStyle = SubtitleSettings.BackgroundStylePref.defaultValue
     margin = SubtitleSettings.Margin.defaultValue.toInt()
+    edgeThickness = SubtitleSettings.EdgeThickness.defaultValue.toInt()
 }