ThreadWeaver-related SqlQueryMaker crashes

As described on Bug #492406, there are still some underlying crashes when doing heavy collection filtering.

My way of reproducing:

1. Type in something to collection filter field.
2. Change collection grouping (from e.g. "Album artist / Year - Album" to "Genre / Album artist / Album")
3. Might crash, usually at least after a couple of tries.

EDIT: tried just now, not able to crash at the moment

Seems to be related to ThreadWeaver::JobPointer deletion handling / something similar, introduced during early stages of Qt5 port (some more details available in email archives).

Partial fix for related crashes was done in e3a14e6ed3aeb0800df2a781b5c999d71e4d524c, but doesn't perhaps fix the underlying issue, just treats some symptoms. Proper fix will possibly need some re-work of ThreadWeaver-related code.

I plan to have a look at this after #8 (closed), but should someone else want to take a stab before that, here is some background info for this.