Fix #20724 - Remove "Hide/show naturals" in keysig context menu.

In addition to the score-wide style (which is kept without changes), naturals for individual key sigs are controlled by a previous section break: if the previous measure has a section break, naturals are always hidden; otherwise they follow the score settings.

The KeySig::_showNaturals member var is removed, as well as all the code involving it. This should bring a substantial simplification of the matter, while keeping the possibility to control individual key sig instances.

A partial regression is introduced for 1.x scores, which did not have the concept of section break: they will show naturals originally hidden, until the proper section breaks are not added (which it likely to be done anyway, if the score contains several pieces).
Merge pull request #780 from mgavioli/Fix_20724_natural_in_keysig_context_Menu

Fix #20724 - Remove "Hide/show naturals" in keysig context menu