Add option to use Inform from an external directory

[DavidKinder]

The MacOS front-end has the option "Use external Inform Core directory" (under Advanced in the preferences dialog). When enabled, this causes the front-end to use the Inform core from a separate directory, so if you're compiling Inform from source you can use your build without having to copy it into the front-end.

On Windows this option will also be added. When enabled there will be a directory picker to specify the Inform Core directory.