[GSoC 2019] Instrument change improvements

[dmitrio95]

This pull request contains changes from #5128, rebased to current master and adapted to work correctly with undoing/redoing actions. The following features from #5128 are not included to this PR:

• Inspector changes, as a more complex Inspector redesign is going to happen soon;
• Instrument change warnings, as this feature has certain issues which make it not suitable for merging at the moment;
• Staff type changes from normal to percussion staff, as this feature seems to have been tested less extensively, and it is unclear whether it should actually belong to instrument change implementation (with the current paradigm StaffTypeChange elements should probably serve this purpose better);
• Tests: some tests still need to be added, but tests from [WIP] Instrument change improvements #5128 cannot be reused directly since they assume the presence of "instrument change warning" feature.

The changes from this pull request might be adjusted in the future but still provide some initial version of what could potentially be merged to master regarding that feature before the next releases.

[MarcSabatella]

Looking forward to checking this out! FWIW, it's introduced a warning, an unused variable "seg" in Score::deleteItem()