- Created a new detail namespace
- Moved shared module implementation details into the detail namespace
- Split dependencies so source files only rely on details in the detail namespace
- Removed all code from the juce_gui_basics.cpp file

The biggest new feature in this commit is the addition of
NativeMessageBox::scopedAsync and AlertWindow::scopedAsync, both of
which return an instance of ScopedMessageBox that will hide the message
box in its destructor.

The code for displaying modal dialogs on Windows has also been updated.
Now, the dialog itself is run from a new thread with its own message
loop. This means that when the dialog is dismissed, the background
thread can be joined safely. In plugins, this means that there's no
danger of the plugin view being destroyed from within the message box
runloop, for example.

The warnings are only silenced in code that is conditionally used on
older platforms. When the newer (non-deprecated) API is available, it
will be used.

The current minimum-supported Xcode (10.1) includes the iOS 12.1 SDK, so
APIs from iOS 12 will always be available.

The current minimum-supported Xcode (10.1) includes the iOS 12.1 SDK, so
APIs from iOS 10 will always be available.

The current minimum-supported Xcode (10.1) includes the iOS 12.1 SDK, so
APIs from iOS 10 will always be available.

This fixes an issue where building with an old deployment target would
cause apps to display in "unsafe" screen areas.

- Add AlertWindow::show() and showAsync() methods that take a MessageBoxOptions argument
  - Add NativeMessageBox::show() and showAsync() methods that take a MessageBoxOptions argument
  - Update the DialogsDemo to demonstrate the new methods
  - Deprecate AlertWindow::showNativeDialogBox() in favour of the NativeMessageBox methods
  - Pass button strings specified in MesssageBoxOptions to native dialog boxes correctly
  - Use modern TaskDialog on Windows for the native dialog box where available