[REQ] Split file enhancements

[Maxwish]

Request: "Split file" enhancements

1. When combining files: check if there is enough space on the destination drive before starting combine action. Should be easy to implement as the total file size is already mentioned in the .CRC file.

2. Add option to verify the stored CRC32 of split files only.
Could be implemented by added support for TC's splitfile .CRC file in the "VERIFY CRC CHECKSUMS" command
or
by adding a checkbox "Check CRC only" in the "COMBINE FILES" command.