SZX support, BASIC attribute editing and PAWS import.

New features:
o Can now modify BASIC attributes for supported files (Eg, file type, Code load address, ect)
o SZX file support
o Added form to import and convert the vast majority of 48k PAWS text adventures.

Bug fixes:
o TR-DOS initialisation error fix stopping being able to read files.
o Some error messages corrected
o Problem with files < 128 bytes long fixed.
o Sector list generation corrected for MGT entries
o Corrected error handling for Bad variables when parsing basic
o Corrected various errors when converting basic text-> encoded basic preventing some files working.
o Added Tape file types to an ID array so they don't appear as INVALID despite working.
o Added variable name decoding for +3 files.
o Better handling of "header" types that are not actually headers.
o Lots of comments updated
o IM fix when converting Snapshots to loadable files
o Fix for Z80 files, wasn't decoding SP.