Add new type of optional tokens in format string (Collection Organizer) - "[]"...

Add new type of optional tokens in format string (Collection Organizer) - "[]" square brackets. Syntax: [%token%: any formatted string], or Its short format: [%token%]. If you specified replacement string (string after colon ":" ) empty token will be replaced with this string (replacement can also include tokens and optional tokens); If no replacement defined default value will be used (e.g. Unknown artist for %artist%).
Split QStringx.h on .h and .cpp because It became huge.
BUG: 264874