Title: Sleep timer options: persistent duration and start on boot.

This patch adds two sleep timer options and a sub-menu to hold them (Settings > General Settings > System > Sleep Timer).

The two options are;

Sleep Timer Duration: Allows the value to be set permanently (as 'sleeptimer duration' in the config file). The value will be used until reset to a new value.

Start Sleep Timer On Boot: Whether you would like a sleep timer to automatically be started when the device boots up (stored in the config file as 'sleeptimer on startup').

These options are particularly useful for users who have a dedicated DAP by their bedside, or a specific config file used when sleeping. When the later is set, powering the device up will play for the users specified duration, then switch the device off.

The existing (System > (Date & Time) > Sleep Timer) option has been modified to be a switch showing "Start Sleep Timer (duration)" when inactive, or "Stop Sleep Timer (remaining)" when active. Duration and remaining being the current setting and remaining minutes left on the timer, respectively.