Upgrade "parts" functionality

Suggestion: add drop-down menu with the list of instruments to the toolbar. When user selects an instrument, this instruments’ part opens for editing. After editing the instrument user can close the part (or leave it open if he doesn’t feel like closing it), and the changes will be saved both to the part and to the whole score.

When the part opens, if something (for example, some bar) was selected in the score, it will be selected in the part as well and the view of the part will be on the page where the selected element is situated - extremely helpful for editing purposes.

I also suggest that the same thing (some instrument part opens for editing) happens when you double-click some empty space of the corresponding instrument’s staff or ctrl+click or shift+click the name of some instrument on the score.

(See the screenshot attached)

The existing functionality of “creating parts” will be used only for making parts which contain more than one instrument. And, when such part is made, it will appear in the drop-down menu mentioned above.

Having to do parts via "File->Parts" is inconvenient, especially for editing purposes (while you are in the state of arranging/composing some score), so these improvements will make the process much more pleasant, in my opinion.