GUI: Fail-safe file save

[rakhimov]

Upon file save operations,
should the program, system, or IO fail for any reason,
the Model file should never be left in a partial state or be corrupted.

Note that this does not apply to "exported" files,
such as graphics (SVG), print-save (PDF), or report (MEF XML).

Qt provides QSaveFile for this purpose; however, it implies the use of Qt File API -- not C API.
The core code works with C API
and currently doesn't report C stdio write-function failures.

Moreover, the IO operations are blocking, so there should be some status line messages.