Fixes unpredictable or unintentional joystick button ordering by sorting
buttons according to their HID Usage property.  This allows SFML to
adhere to a manufacturer's (or driver implentation's) intended button
ordering.

The coordinate system transforation that occurs when dealing with window
positions on OS X requires (or at least warrants) some fuzzy logic in
dealing with getting and setting window positions.  In any windowing
system, requesting a position should effectively yield the components
for an identity transform of the window's current position.  In other
words, coordinates obtained by a call to getPosition should have no
impact in the window's position when immediately applied to setPosition.

This change brings getPosition and setPosition much closer to acheiving
that identity transform in the OS X implementation, correcting the error
in most cases, and producing far less unpredictable results in the cases
that remain affected.

The coordinate system transforation that occurs when dealing with window
positions on OS X requires (or at least warrants) some fuzzy logic in
dealing with getting and setting window positions.  In any windowing
system, requesting a position should effectively yield the components
for an identity transform of the window's current position.  In other
words, coordinates obtained by a call to getPosition should have no
impact in the window's position when immediately applied to setPosition.

This change brings getPosition and setPosition much closer to acheiving
that identity transform in the OS X implementation, correcting the error
in most cases, and producing far less unpredictable results in the cases
that remain affected.

switching.

The existing fullscreen implementation simply opens a window to the size
of the desktop (while creating a context of the requested mode size).
This change tests the requested fullscreen mode for validity and, if the
mode is a valid hardware mode, switches the main display to the
requested mode.  The original mode is retained so that when the window
is deallocated, the desktop can be restored to the user's normal display
mode.

This could be further improved by offering the option to fullscreen onto
another display, but that requires multi-display support higher up in
SFML as a prerequisite.

Video mode listing has become a responsibility of the DisplayImpl
object, which makes sense because the display itself is the source of
the video mode list.

application window loses focus.

As a consequence of performing a true mode switch on fullscreen window
creation, the desktop will no longer be at the user's normal operating
resolution while the SFML application is running.  This change resolves
that issue by performing a mode switch back to the original mode
whenever the application window loses key focus.

working on the previous change.

This feature branch added a couple files that need to be part of the
Xcode project or it fails to link.