Show whether current session is modified

[iromeo]

If a user saves the current session and then modifies it, it will be nice to tell them that the session has unsaved changes that will be lost on JBR restart. Normally other programs show * symbol at the end of title.

Note: Serialized session info also contain current visible genome range, so such a feature should likely ignore changes in genomic range and focus more on the tracks list, their settings (e.g. colours), order, and panel info. Otherwise, the session will be marked as modified almost immediately after opening because users normally scroll the session left/right in 'read' model. So such asteriks will be always shown and will bring low profit.