End sounds and pausing.

Following suggestion from @BrotherSharper, have added the ability to pause and resume an in-progress timer. This pauses both timer window animations and text countdowns on all clients.

Following suggestion from @oamsden, have added the ability to assign an "End sound" to a timer. This end sound plays at the end of the timer duration. The GM can choose either a pre-existing sound or to use their own sound from an audio file. The GM can also preview the sound when in the Hourglass Options window.

Following the report from @ZheakTheGeek, have updated documentation to indicate the correct location of the Hourglass controls within the "Token Controls" menu item and included a screenshot as additional clarification.

Following bug report from @ZheakTheGeek, have disabled the double-click minimize functionality for Hourglass windows pending fixing of the bug which prevents the windows from being restored.