Fix: Incorrect .app frankified if there are multiple app targets built

### Changes
Frankify the app with the correct name, not the first .app directory in the build output dir.

Change from building directly in frankified_build/ to building in Xcode's default location and copying only the app to be frankified into frankified_build/.

Extract logic into xcodebuild_command.

### NB
We have to parse `xcodebuild -showBuildSettings` to get the product name associated with a scheme and the default build directory. Using the xcodeproj gem would be better, but, as noted in previous comments, xcodeproj doesn't support accessing existing schemes from a workspace as much as it does creating them. 

I've been using this fix myself for the last month or so. Just checked it against the Controls example project; seems okay with targets, too.