Automatic selection in the launcher is disabled for now, since
it needs more testing before release, but the basic UI for selection
is straightforward enough to throw in.
Updating the value takes immediately, as it did in 2017.1 and prior.
Additionally, passing —download-dir on the command-line is detected
and handled specially; the UI option is disabled, and no changes are
made to the value passed in. (The launcher does not override the value
with any value it has saved)
