The "Select Destination Directory" dialog didn't ask for confirmation

The "Select Destination Directory" dialog didn't ask for confirmation
about installing into a pre-existing directory *unless* you hit the
Browse button first.  At least while testing, this screwed me repeatedly.
Plus I really liked the Inno Setup scheme of giving you a list box in
its "select directory" dialog without needing a distinct browse button
to ask for that.

So I redid this dialog from scratch:  now gives a list box at once, the
browse button is gone, it asks for confirmation if the directory already
exists, and, since this is the first dialog in the set now, also removed
its "Back" button.