[mouse-] click on error note to run error-cell

[midichef]

This is a UI change, to let users click on the ! symbol to run error-cell. The goal is to make the error-cell command easier to discover, and easier to get to without remembering the shortcut zCtrl+E or the command name.

A drawback is that users may accidentally click on the !, thinking they're clicking on a cell. After error-cell runs they may not quite understand what has happened. To make it clearer, I've created a new kind of sheet, ErrorCellSheet, with its own guide to explain where they've come to, and how to get out. And in that sheet, the column name will be cell_error, which is more specific than the current text.

What do you think?

[saulpw]

I agree with making the ! symbol clickable. What about wrapping it with [:onclick error-cell], instead of the custom code? We'd have to update sheets.py:881 to use clipdraw instead of addstr, but then it would be configurable via an option (currently note_getter_exc but I think that should be renamed to disp_note_getexc). Also currently clipdraw auto-underlines clickable cells, which doesn't look good, but we could change it so that it doesn't.

[saulpw]

Okay, I renamed options.note_* to options.disp_note_* and made it use clipdraw, as mentioned above. So this PR can be changed to be the changes to the error sheet help text, and updating the options. I do like the idea of making the notes clickable.