Fix white background on preference dialogs.

Fixes #750 (closed).

So it turns out that the ListPreference type (i.e. the one used for the "Theme", "Language", "Automatic update interval", etc) is to blame. Found this SO post which pointed to this reddit post which pointed to this class which seems to solve the problem.

The problem is:

  • The ListPreference class is responsible for creating a Dialog to pop up.
  • It uses the android.app.AlertDialog.Builder, not the android.support.v7.AlertDialog.Builder.
  • As it doesn't use the support library builder, it doesn't get the support library styles.
  • The solution is to use a custom subclass of ListPreference which uses the correct AlertDialog.Builder, and thus it respects out theme choice.

I've included the GPLv3+ licensed AppCompatListPreference from the QuickLyric project. Unsure whether I need to mention that in the README.md or whether just leaving the license header in that file is enough.

These three commits should also be cherry-picked into stable-v0.102.1 after review.