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.

AUv2 plugins on Arm that are hosted out-of-process (e.g. in Logic 10.7)
can sometimes crash due to endlessly recursing through becomeKeyWindow.
This tends to happen when displaying a secondary window in a plugin,
e.g. an AlertWindow, then clicking on a secondary app, then clicking
back on the AlertWindow.

To avoid this case, we check that the peer isn't already key before
calling makeKeyWindow.

Unfortunately, we can't use isKeyWindow to avoid the recursion because
this may not return true until after becomeKeyWindow has returned.

The previous method for finding the dialog's owner was ineffective, and
it was still possible for other windows to obscure the file picker in
many cases. Using GetActiveWindow seems to produce the expected
behaviour.

Without this VoiceOver will iterate over menu items left to right first,
and iteration order will be affected by whether the PopupMenu gets
broken up into multiple columns due to not enough screen space.

Without this event Components such as Button would remain in a hover
state indefinitely after being clicked on a touch screen. The simulated
event matches the behaviour of the other platforms.

This allows paths that are prefixed with environment variables to behave
as expected. This is useful when scanning the default LV2 locations in
the AudioPluginHost on Windows.

Fixes #1118

Previously, for the following snippet, the menu's LnF was incorrectly
being forced to the default LnF. The correct behaviour is to display the
menu using LnF v4. The menu doesn't have an explicit LnF set, so it
should use the LnF of its parent component.

    LookAndFeel::setDefaultLookAndFeel (&lookAndFeel_V1);
    setLookAndFeel (&lookAndFeel_V4);
    PopupMenu().showMenuAsync (PopupMenu::Options{}.withParentComponent (this));

When the mouse moves from one window to another, the mouseExit event
for the old window can occur after the mouseMove event for the new.
Until this commit this would cause the MouseInputSource to be
associated with the old window, and getComponentUnderMouse() would
incorrectly return a nullptr.