Relocate libopenshot.log recovery code to exceptions.py

[ferdnyc]

The MainWindow method create_lock_file(), of all things, has been repeatedly flagged as a high-complexity function. (One of four in windows.main_window, the remaining three being MainWindow's findAllMarkerPositions, keyPressEvent, and most disturbing of all, __init__.)

In the case of create_lock_file(), the complexity came about because, right in the middle of doing its job (creating a lock file), it went off on this long, completely-unrelated tangent where it would parse the end of the last session in libopenshot.log looking for stack traces.

Since that has nothing to do with creating a lock file, I pulled that entire tangential block of code, plus the tail_file() function it (exclusively) made use of, out of windows.main_window entirely. Instead I converted them both to functions in classes.exceptions. (Not a perfect spot, but a much better one.)

Theoretically now, exceptions.libopenshot_crash_recovery() could even be called by app.py instead. Like it probably should be.

(There's still too much stuff in MainWindow that's really application-startup stuff, IMHO. I'd remind any skeptics that, again, MainWindow.__init__() is flagged for excessive complexity...)

[jonoomph]

LGTM!