change timeformat hh:mm:ss to not return AM/PM anymore, introduce 'h:mm:ss xx' and 'hh:mm:ss xx' for that purpose

This one puzzled me a bit the last day or two:

Tried to retrieve the system time via the System.Time() label in python, but always in hh:mm:ss format with leading zero where needed. That worked as expected when setting XBMC to some 24h clock format region (e.g. "Germany" - "13:23:45"). When switched to a 12h region ("English (US)"), it suddenly returned "h:mm:ss xx" (i.e. "1:23:45 PM"), where it was expected to return "01:23:45" for that format specifier.

This PR changes that so "hh:mm:ss" returns a time formatted in the expected way (and not behave like an empty format string was given resulting in guessing), and additionally adds formats "h:mm:ss xx" and "hh:mm:ss xx" to have format specifiers including am/pm. 

Grepped through my (small) list of system/user addons, none seems to use that. GUI (system time display, movie duration/length, EPG) seems unaffected.