The transaction dialogs have a habit of randomly disappearing,
because of garbage collection. This was particularly common
if you viewed the details of a tx in your history immediately
after electrum startup, or after pressing Broadcast.
Other tweaks:
- Distinguish saved and broadcast.
- When signed, consider unsaved and prompt to save if not
subsequently saved or broadcast.
- Hide broadcast button after broadcast.
- Hook into the closeEvent so closing the window with the mouse
has the same effect as pressing the close button.
3446e1fd56