Add keycode definitions required for Apples Expose & Dashboard keys (F3/F4)

Add keycode definitions required for Apples Expose & Dashboard keys (F3/F4)

Apple's Expose and Dashboard keys (F3 and F4) have correct keycodes
reported by evdev, but are not mapped.  This adds the required keycodes
and symbols, using the symbols XF86LaunchA and XF86LaunchB, which allows
the keys to be used in compiz configuration.

(This patch has been included in Ubuntu since 2010.)

Bug: #34351
Ref.: https://bugs.launchpad.net/ubuntu/+bug/520519

Signed-off-by: Bryce Harrington <bryce@canonical.com>