#1941, 1948 - Fix possible warnings in movie and graphics helpers, add functions to convert between seconds and hh:mm:ss.dd (and their unit tests).

Modifications in the fix for #1928 could cause warnings if error reporting is enabled. In production they're disabled anyway, but this should be cleaned up nevertheless.

Added helper functions to convert between a number of seconds and hh:mm:ss.dd (the format ffmpeg likes to use).

graphics helper: added "@" before unlink
movie helper: added three issets
movie helper: changed time formatting to avoid possible warnings and/or issues with the is_dst argument of date()
movie helper: added new helper functions for converting to/from hh:mm:ss.dd