Validate all playlists

Returns a list of all playlists with any validation errors, total item counts, and total duration. Each entry is checked the way fppd will play it: sequence, media, image and script files must exist; sub-playlists must exist and be valid themselves (and not include themselves); commands must still exist in fppd, and command arguments naming a playlist, sequence, media file, effect, script or command preset must name one that exists. Disabled entries are not checked, since fppd skips them. messages is a flat list of human-readable problems; issues carries the same problems with the section and 0-based index of the offending entry (index is -1 for a problem with the playlist as a whole).