Implement sound framework

[albireox]

Observers have requested having some form of sounds feedback in Boson. PR #28 tracks this implementation.

Needed features are:

• Allow to play sounds on certain events (error message, exposure completed, exposure failed, etc.)
• Create a library of sounds, both sound effects and live/generated voice that can be used for sound events.
• Allow three modes of operation: on in which all the defined sound events play a sounds, minimal in which only certain events play a sound (and always sound effects, not voices), and off where no sounds are played.
• Add a quick muted option with a shortcut to temporarily disable sounds (this is different from the off mode).
• Assign sounds to sound events and allow to change in configuration file.

More advanced features, not critical at the beginning are:

• In Preferences, allow users to select what sound effect / sound file is played on each sound event.
• Allow users to use sound files from their host system.
• Use text-to-speech to allow users to define sound effects from text (see here).