Right-clicking on column header resulted in this error:
TypeError: could not convert type EventButton to GdkEvent required for parameter 0
The following fixes and cleans up the issue:
- Move the signal creation to the class, using the __gsignals__ dict.
- Replace `Event` with `object` since we are passing an EventButton as
Gtk3 no longer accepts it as an Event.
- Replace deprecated menu `popup()` with `popup_at_pointer()` which also
fixes a critical gdk error when using `popup()`.