Revert "vteapp: Three-state bell option"

This reverts commit 983b3840454267205ce789498f8d0941b41c8d60.
Revert "widget: Implement visual bell"

This reverts commit 4add1f726a8acef0525f5903b9cbe7ae0b905658.

https://bugzilla.gnome.org/show_bug.cgi?id=602726#c19
widget: Remove broken visual bell support

https://bugzilla.gnome.org/show_bug.cgi?id=602726#c19
vteapp: Make the bell option consistent between the two test apps
widget: Rename beep signal to bell

https://bugzilla.gnome.org/show_bug.cgi?id=602726#c23