While it's still session-limited, this means you don't have
to restart to get your locking ability back.
Finer-grained locking enablement probably makes better long-term
sense, but this at least prevents "gun shyness" about a setting
you can't turn off again easily.
Also it gets a bool's-worth of application configuration state
out of a dialog (it was a static).
Calling wxDialog::EndModal() directly from inside the dialog is a
potential bug if the dialog is shown in the modeless or window modal
(quasi-modal). Use the internal button events where possible and
check for the appropriate mode before calling the correct end dialog
function.
This unifies everything under a single architecture with a "don't
show again" dialog. Since everything now goes through the same
path it should be reasonably easy to make it do whatever we want
in the future.
Right now it presents 3 options: modify only unlocked items, override
locks and modify all items, or cancel command.
