Allow custom sound for DankPomodoroTimer

Due to the plugin using the default notification sound, it is really easy to miss it. Allowing for custom sounds would be a helpful addition as it will make the timers more prominent.

Start by locating the DankPomodoroTimer plugin and inspect how it selects the notification sound. Determine where a sound preference can be exposed and used by the timer. Done means the plugin can use a custom sound instead of only the default notification sound.

Written by the indexing model from the issue text.