The new class hold a list of ChildProcesses and periodically checks
their return value until they report termination. On Linux this check
is necessary to avoid leaving zombie processes behind.

This also fixes an issue where MIDI CI header data could contain spaces,
which is not allowed according to the spec.

This allows real-time threads to join an audio workgroup on Apple platforms.

- added unit tests
- best performance timers used for each platform
- fixed an issue in which timer callbacks could drift

This commit adds the JUCE_DECLARE_SCOPED_ENUM_BITWISE_OPERATORS macro, enabling bitwise operators on scoped enums. This feature simplifies the use of scoped enums in situations where bitwise operations are needed.

This patch also makes Optional public to end users.

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.