Improvement: retain audio settings

[pega3]

Currently, the application does not remember that I paused the audio when a game that I have launched exits.
To reproduce:

• Run ESLauncher2
• Pause Audio
• Launch Endless Sky game
• Exit game
  => Launcher will play music again.
  It should retain the setting before the launch, i.e. if I paused the music, it should not start playing again.

To take this a stop further, ESLauncher2 could as well retain this setting between executions of ESLauncher2 itself. But that would mean the state has to be persisted.

[MCOfficer]

Let's keep it simple for now and do the rest after #35

[MCOfficer]

f2e3d03