Fix #501 Warn user about losing new talk info

When the user switches from one talk to another or creates a new talk
when the currently selected talk has unsaved changes, a window comes up
asking the user if they wish to save their changes, with the options
"Yes", "No", and "Cancel". "Yes" saves the changes and selects the new
talk; "No" selects the new talk without saving the changes; and
"Cancel" keeps the current talk selected without discarding or saving
any changes made.

Fix #501
Close #605