Replace obsolete delete_event signals

We can use `response` on dialogs or `destroy` on windows.

Also, using `response` means we don't need to _also_ connect to `clicked` on action buttons.

Testing Done:
Compiled and opened 'Get Buddy Info' then closed with titlebar button and Close button, and confirmed that callback was triggered. No tests for the other UI.

Reviewed at https://reviews.imfreedom.org/r/2189/