Managing external libraries - including VSTs - MU4

MU4 allows more libraries to be used than MU3 which leads to greater flexibility and opportunities for better sounds, but also more chances of things going wrong.

Some libraries can work well with external drives, such as SSDs, and that may be prefered. Examples are some of the Spitfire Audio libraries.

Problems can then arise if for some reason the external libraries are not mounted. Arguably (??) this is not a MuseScore problem, but some features - if possible - might be helpful.

1. An indication if any external libraries required for a score are not working or available.
2. An alternative library to use if the preferred library is not available. Indeed this could be extended to several "alternative" libraries, to ensure that any score always produced some output if any individual libraries or instrument patches are not accessible.

Something to ponder on, anyway.