[atv] added maxplayerforwardrate to advanced_settings

Hi,
Limiting forward/backward seek speed according to settings (new option) in advancedsettings.xml.
Seeking at more than 4 times of playspeed on SMB-shares, don't work very well for us.

I just wonder if the speed setting is video only (because i put it into video-subtag of advancedsettings)?

Greets wolfi