AppKit: add NSPopover notifications

Consolidate Notification EventArgs augmenting classes into
EventArgs.cs (similar in spirit to Enums.cs) since this is
going to be a common pattern.
AppKit: more notifications

- NSScroller:
  - NSPreferredScrollerStyleDidChangeNotification
- NSView:
  - NSViewFrameDidChangeNotification
  - NSViewFocusDidChangeNotification
  - NSViewBoundsDidChangeNotification
  - NSViewGlobalFrameDidChangeNotification
  - NSViewDidUpdateTrackingAreasNotification
MonoMacGameView: use NSView.GlobalFrameChangedNotification
AppKit: make a number of types partial
NSImage: add CGImage property to match iOS
NSTableColumn: add explicit NSString constructor

Fixes an API break when the System.String constructor
was added where there was an ambiguity between the
NSObject constructor and the implicit conversion from
NSString to System.String.
Include full namespace in error message about Class.ThrowOnInitFailure.

Hopefully avoids confusion like the comments here: http://stackoverflow.com/a/15524950/183422
Remove NSMenuView initAsTearOff as it's not supported on OSX (and it's managed/native signatures did not match anyway)
Add new NSUrlCredential.cs file in build
Add missing [NullAllowed] attributes in NSLayoutConstraint [#10984]
Fix monomac to work with async.
NSGraphicsContext needs to be marked with [ThreadSafe] since CurrentContext returns a context that is valid for the current thread. Also register NSPrintPreviewGraphicsContext or it won't be possible/easy to use the context for the print-preview. ref: desk #30220
Do not reset MCS if one was already supplied
MonoTouch.Constants and MonoTouch.MonoTouchException are not namespaces (and can conflict with types when compiling using newer compilers)
Preferences Window Sample

Sample project to demonstrate how to implement preferences window as
seen in many Mac applications.
Update README.md
Merge pull request #108 from taimila/master

Preferences Window Sample
Allow HAVE_ASYNC= as a make argument instead of checking the system mono version.
Add support for marshalling native exceptions to Xamarin.Mac.
Marshal native exceptions for NSColor.BlackComponent.