Since the returned object is now owned by the callee object, we can
simply vend a ChildType&. This allows us to use "." instead of "->"
at the call site, which is quite nice. :^)
The selected option was stored in a captured stack variable which was
long gone by the time we looked at it, so this dialog didn't really
behave the way you'd expect. Put it in a member instead. :^)
Now it actually defaults to "a < b" comparison, instead of forcing you
to provide a trivial less-than comparator. Also you can pass in any
collection type that has .begin() and .end() and we'll sort it for you.
I probably would've done INI config removal in another commit, but it
fit well here because I didn't want to pledge wpath for SystemMenu if I
didn't need to.
Frankly, that's something that I think should be done: allow ConfigFile
to be used read-only.
This will allow us to run the system menu as any user. It will also
enable further lockdown of the WindowServer process since it should no
longer need to pledge proc and exec. :^)
Note that this program is not finished yet.
Work towards #1231.
