Allow the user to edit how many lines a staff has

If a Neon staff has four lines, but the real staff has five, we need a way for the user to add a staff line without having to delete the entire staff (and all the information on it) to input a five-line staff instead.

When a user selects a staff, staff-specific editing prompts appear in the "Edit" box. A prompt could be added between "Split system" and "Column information" that would say "Edit number of lines", with a cute little dropdown menu with numbers from 1 to 7 or something.

@annamorphism what's the most lines you think a staff could have?

Start by tracing the staff-specific prompts shown in the Edit box, especially “Split system” and “Column information,” and identify where staff selection and line counts are handled. Done means a selected staff can change between supported line counts, including adding a line, without deleting the staff or its existing information.

Written by the indexing model from the issue text.