Backup and restore corrupted guisettings.xml

Guisettings.xml is heavily used by xbmc (many read/write operations), hereby this file easely gets corrupted when xbmc crashes,OS crashes, power cut,...

This sucks big time as xbmc will recover guisettings.xml to its defaults!

With this PR: A backup from guisettings.xml will be made each time xbmc closes and the settings will be restored from this backup if necessary.

Very usefull for the raspberry pi, as most people just pull out the power cord and risks corroption (guisettings.xml)  by doing that.