Add titleNode prop to @jbrowse/core/ui/Dialog

[cmdcolin]

This allows users to provide instance of a ReactNode, e.g. render a custom component into the title area of the Dialog

I used this in a plugin to provide a "Help button" next to the title

I avoided actually allowing the "title prop" to be a ReactNode, because the title specifically a string in MUIDialogProps, and it does not like being overridden with something different. While that type is not necessarily 'sacrosanct', it seemed like it was better to just extend it with a new prop, named titleNode, instead of replacing title with something different.

note that changing title prop would also not be backwards compatible either, because a titleNode would not pass older versions of JBrowse passing title through the SanitizedHTML, so by doing things this way, it is a bit more backwards compatible if components supply both title and titleNode

I used the term titleNode to refer to the type React.ReactNode

[cmdcolin]

example screenshot of a dialog title using the custom titleNode with help button

without this PR, users would either have to replace the entire header with a custom component or just settle with title being a plain string, which wouldn't be able to add e.g. a custom help button