With this patch applied, the DemoRunner should build under MinGW, and be
(nearly) feature-complete compared to the MSVC build.

Specifically, when building with MinGW:
- Adds support for accessibility
- Fixes build issues in the juce_video module
- Fixes a link issue in the VST3 wrapper when VST3_CAN_REPLACE_VST2 is
  defined
- Adds support for the new-style native FileChooser
- Tidies up some other low-severity warnings

Known issues:
- Direct2D rendering is still not supported when building with MinGW due
  to ABI compatibilities.

Previously, with two instances of the MIDILogger AUv3 in series in
Logic, the timestamps received by the second plugin in the chain would
not match the timestamps of the events emitted by the first plugin.

The 'wrapper' accessibility handler is now ignored if a menu item has a
custom component, and has no submenu, and cannot be triggered
automatically. This avoids the case where a custom menu item may end up
with a wrapper accessibility handler that has no useful actions.

This patch also adds a 'label' argument to the addCustomItem functions,
which allows text for the screen reader to be supplied in the case where
a custom component is in use, but the menu item has accessibility
actions.

Effects of this change for windows with native titlebars:
- Resizable windows without maximise buttons will now only allow the
  'zoom' behaviour from the rightmost titlebar button (i.e. this button
  will not allow entering full-screen mode)
- Non-resizable windows will grey-out the rightmost titlebar button,
  whether or not the maximise button is enabled

This works around a rare issue in the DialogsDemo where the AlertWindow
would fail to display because no peer had focus.