Sometimes, changing the bounds of the window inside the DPICHANGED
handler can cause further DPI change events to be processed. Previously,
the scaleFactor set by the "inner" events was also being used when
notifying listeners about the "outer" events, leading to graphical
glitches.

An effect of the bug was that VST2 views in the AudioPluginHost would
occasionally render with an incorrect size and position after dragging
them between displays with different scale factors.

With this change in place, we only notify listeners and update window
bounds once there are no DPI changes in progress.

- 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

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.

In cases where internalClickCallback() modified the state of the button this was not being updated until the next call to updateState(). Calling it a second time after the click callback will make sure the state is up to date.