- 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 current minimum-supported Xcode (10.1) includes the macOS 10.14.1
SDK, so APIs from macOS 10.14 will always be available.

The current minimum-supported Xcode (10.1) includes the macOS 10.14.1
SDK, so APIs from macOS 10.13 will always be available.

Ideally, we want to pass shortcut keys to the component to handle, and
only fall back to invoking a menu item if the component was unable to
handle the keyboard event, or if the action was triggered by
clicking/selecting an item in the menu itself. The old implementation
tried to work out whether the action was triggered by a shortcut by
checking the event's characters and modifiers. This method was
inaccurate, because some shortcuts (such as arrow keys) may add
unexpected numpad/function modifier flags.

We now try handling shortcut keys directly in the peer, and pass events
up to the superclass (which will forward them to the main menu) if the
event could not be handled.

This commit also adjusts some Objective-C method signatures to use the
correct string encoding for the BOOL type.

This fixes a regression introduced by
6e9261ea66 which meant that components
were not given a chance to respond to shortcut keypresses if those same
keypresses were registered for a menu item. This resulted in behaviour
where shortcuts such as 'cmd+c' would not be passed to a focused
TextEditor if a different command with the same shortcut was registered
in the main menu.

With this change in place, we now check whether the menu item's shortcut
keys match the current event's pressed keys. If the keypresses match, we
can assume that the event was triggered by the keyboard, and dispatch
the keypresses to the ComponentPeer. If the keypresses do not match,
then the menu item was likely selected using space/return, or by
clicking, in which case the event is dispatched directly to the
ApplicationCommandManager.

Previously, when a file dialog was launched synchronously, the 'dummy' modal
component was preventing user input events from reaching the preview component.

Now, we explicitly allow input events to reach the file preview component, if
it exists.