Added pop up error message if user attempts to excute assembly file without having selected one first. This prevents unpredictable behaviour, since the swing worker thread isn't created unless a file has been chosen.
TextFileFilter extends FileFilter; a filter added to the FileChooser object in CPUframe to restrict the choosable files to text files only (prevents user from opening incompatible files).
Added restrictions to selectable files.
Added restrictions to selectable files.