Allow dragging icon onto system files to launch as interpreter

[inexorabletash]

Use case: Allow launching "interpreters" (system files that follow the ProDOS interpreter protocol) with a target file, without hard-coding an association in DeskTop or introducing more UI surface. For example, dragging text files onto EDIT.SYSTEM.

What's required:

• Modify icontype_iconentryflags_table entries for system and application to have kIconEntryFlagsDropTarget
• Fix any places that abusekIconEntryFlagsDropTarget as "is this a directory type?" there's at least one
• In HandleClick::_IconClick, after a successful drag onto a target icon, if the target is a SYS file then:
  • If any modifier is down then fail (reserve for future)
  • If GetSingleSelectedIcon is 0 or trash_icon_num then fail
  • Call GetIconPath; if Z=0 then fail
  • Call SetOperationDstPathFromDragDropResult; if C=1 then fail
  • OPEN file at path operation_dst_path; if C=1 then fail
  • READ 5 bytes and CLOSE; fail if C=1 or trans_count < 5
  • If bytes are not $4C x x $EE $EE then fail
  • Copy operation_src_path to src_path_buf
  • Copy operation_dst_path to INVOKER_INTERPRETER
  • Tail call LaunchFileByPathImpl::launch

This is an alternative to #192 that involves less UI creation.