English spelling corrections English/strings.po

Fixes in strings.po as per Martijn, English UK (en_GB) cannot be added to Transifex as the main xbmc English source is UK English.

Contains corrections for synchronization to synchronisation, initialize to initialise, however virtualization is correct and remains with the letter z.

Program has been fixed contextually i.e. using 'program' in computing contexts, and 'programme' everywhere else (e.g. computer programs, television programmes)