GtkFilechooserWidget: prevent oblivious selection of file
which could happen after confirming the "file overwrite" dialog and may result in a different file being overwritten causing data loss. The oblivious file selection can be done by a mouse click or keyboard press sent inadvertently just after confirming the "file overwrite" dialog (and before the enclosing GtkfilechooserDialog is closed). Fixed by adding a flag to ignore any button/key press events sent to the file list. We set this flag just after the user accepts the "file overwrite" dialog (which means the enclosing GtkfilechooserDialog is about to get closed). And we restablish the flag when the dialog is shown again (in its map() handler).
Fixes data loss issue #2288 (closed)