The default main JuceActivity for JUCE apps calls appNewIntent() in its onNewIntent() implementation which is defined in juce_gui_extra. This commit moves the implementation into juce_gui_basics.

If an aiff file advertied a large number of metadata keys, the
StringPairArray::set calls resulted in quadratic complexity.

We now read keys into a std::map instead, as it enables much faster
lookup, and then convert back to a StringPairArray at the last possible
moment.

This change allows multiple versions of JUCE to be installed to the same
CMAKE_INSTALL_PREFIX, avoiding conflicting paths to juceaide.

Previously we relied on Foundation.framework being implicitly linked by other frameworks, but this change is required when building with Xcode 12 targeting older macOS versions to fix missing CFNetwork symbols.