Draw a "real" title bar gradient when possible.

Drawing code is also now modularized into several methods.
Remove the need for IN_COMPILING_LION.

This allows ALL applications to use Lion features regardless of whether
they were compiled with a 10.7 or later SDK.
Resolve some memory leaks in MRC mode.
Use CoreUI to draw the title bar and separator if available.

This uses private APIs and so is disabled by default, but for developers
who are not releasing their apps in the App Store (or have a separate
non App Store build), INAPPSTOREWINDOW_USE_COREUI can be set to 1 to
draw 100% native title bars with graceful fallback to custom drawing if
the CoreUI drawing function could not be loaded.