Previously, individual components had to ask the peer to hide and show
the keyboard, by calling textInputRequired() and
dismissPendingTextInput() respectively. When an onscreen keyboard (OSK)
was required, most Peer implementation would directly hide/show the OSK
inside these function. However, the iOS ComponentPeer implementation
instead listened to the application's global keyboard focus, and only
opened the OSK when the focused component was also a TextInputTarget
with active input.

The iOS scheme seems like a better design, as it enforces that the OSK
hiding and showing is synced with the keyboard focus of the application.
In the other implementations, it was possible for a Component to call
textInputRequired even when it didn't have the keyboard focus, putting
the application into an inconsistent state. The iOS scheme also makes
the TextInputTarget interface more useful, as it enforces that the OSK
will only display for components that implement TextInputTarget, and
return true from isTextInputActive().

This patch changes all Peer implementations to match the iOS
implementation, improving consistency. Each time the global keyboard
focus changes, refreshTextInputTarget is called automatically, and the
OSK is shown if the focused component is a TextInputTarget that returns
true from isTextInputActive, and hidden otherwise. Components can also
call refreshTextInputTarget manually. This should be done whenever the
component updates the return value of isTextInputActive(). Effectively,
the Peer is now responsible for keeping track of the focused
TextInputTarget, rather than allowing individual components to hide and
show the OSK at will.

Additionally, this patch adds an option to the TextEditor to
automatically dismiss the OSK when the mouse is clicked outside of the
editor. This should improve user experience on mobile platforms, where
touches on sibling components may cause a TextEditor to gain keyboard
focus and unnecessarily display the OSK.