Frequent different crashes when closing on Mac OS X

As it was unclear whether #2633 is supposed to be reserved for a *specific* instance of a crash at close, I thought I'd create a new issue since it appears there may be multiple crash-at-close causes.

I get crashes on close around 75% of the time on OS X 10.8.5 / Qt 5.5.1 / LMMS 1.2.0-rc2.x master but they don't appear to prevent the settings from being saved or have any other negative effect other than inconvenience.

I wanted to get an insight into the common aspects of the crashes so wrote `crashlytics.py` as a starting point for summarising the `~/Library/Logs/DiagnosticReports/lmms_*.crash` files (it's slightly but not entirely re-inventing the wheel): [`crashlytics.py`](https://github.com/LMMS/lmms/files/785859/crashlytics.py.txt)

I've attached two summaries ("A" has 1 line of backtrace, "B" has 5 lines of backtrace) that are the current output of the script:
* [lmms-crash-report-summary-20170220A.txt](https://github.com/LMMS/lmms/files/785857/lmms-crash-report-summary-20170220A.txt)
* [lmms-crash-report-summary-20170220B.txt](https://github.com/LMMS/lmms/files/785858/lmms-crash-report-summary-20170220B.txt)

Summary of the summaries:
```
Total reports processed: 20
EXC_CRASH (SIGABRT) | Occurrences: 5
EXC_BAD_ACCESS (SIGSEGV) | Occurrences: 13
EXC_BAD_ACCESS (SIGBUS) | Occurrences: 2
```

Ideally the script would help identify common backtraces more than it does at the moment but it's at least a helpful way to get a rough summary.

(`QObjectPrivate::deleteChildren()` does seem to occur frequently.)

Feel free to close this issue until more details get added by me or someone else.

(I also installed and ran Valgrind which highlighted some issues but I've not investigated them because as usual I'm trying to do too many things at once. :) )

Start by reviewing the attached crashlytics.py script and the two crash-report summaries, then compare the reported backtraces around QObjectPrivate::deleteChildren(). The issue does not identify a source file or reproducible code path; done would require isolating the close-time crash cause and confirming the fix against the reported macOS failures.

Written by the indexing model from the issue text.