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)