Score and System destructors conflict with each other

While experimenting with PR 4166 (do merge it in time, please), I’ve found that trying to delete a MasterScore causes uninitialised memory access (visible in Valgrind, but occasionally causes later program crashes).

I’ve tracked this down to System::~System and Score::~Score both trying to access measures (MeasureBase) which they both have in a list. The problem is, the one that runs first deletes them, while the second one still accesses them. They have different lists, of course…

This is new in 3.x — in 2.x, only one of them dealt with MeasureBases, hence no conflict.

I think the problem comes from the following:

A score has a number of MeasureBases, which are grouped into systems. However, the score also has a list of them, instead of always accessing them via the systems. The system destructor does not destroy the measures, merely sets their system reference to nullptr.

The fix is likely to be complicated and involve restructuring a lot. ☹