Fixed Issue #1174, improved documentation, and added a debugging helper tool

The fix of issue #1174 is simply an implementation of the suggestion made by akode on that issue's comment thread.

Changed the spelling of the warning message "This file has changed on disk" to match standard English usage ("disk" not "disc" for computer-related things).

Added `testing.py` in `ninja_ide/tools`, with the function `debug_trace()`, an easy way to create breakpoints which work in Qt.