[CTFont] Subclass NativeObject + numerous other code updates (#13102)

* [CTFont] Subclass NativeObject + numerous other code updates

* Subclass NativeObject to reuse object lifetime code.
* Enable nullability and fix code accordingly.
* Use 'is' and 'is not' instead of '==' and '!=' for object identity.
* Use CFString.CreateNative/ReleaseNative instead of other means to create
  native strings (the fastest and least memory hungry option).
* Use the null-safe NativeObjectExtensions.GetHandle extension method to get
  the handle instead of checking for null (avoids some code duplication).
* Use 'nameof (parameter)' instead of string constants.
* Use the 'Runtime.GetNSObject<T> (IntPtr, bool)' overload to specify handle
  ownership, to avoid having to call NSObject.DangerousRelease manually later.
* Use Array.Empty<T> instead of creating an empty array manually.
* Add an NSArray.ArrayFromHandle overload that releases the handle if requested (and use it).

* Remove more code.

src/CoreAnimation/CATextLayer.cs

@@ -90,7 +90,7 @@ namespace CoreAnimation {
 				var handle = _Font;
 				nint type = CFType.GetTypeID (handle);
 				if (type == CTFont.GetTypeID ())
-					return new CTFont (handle);
+					return new CTFont (handle, false);
 				else if (type == CGFont.GetTypeID ())
 					return new CGFont (handle, false);
 				else if (type == CFString.GetTypeID ())

src/CoreText/CTFont.Generator.cs

@@ -24,57 +24,10 @@
 // OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
 // WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 //
-using System;
-using System.Collections.Generic;
-using System.Runtime.InteropServices;
 
-using ObjCRuntime;
 using CoreFoundation;
-using CoreGraphics;
-using Foundation;
 
 namespace CoreText {
-
-	public partial class CTFont : INativeObject, IDisposable {
-		internal IntPtr handle;
-
-		internal CTFont (IntPtr handle)
-			: this (handle, false)
-		{
-		}
-
-		internal CTFont (IntPtr handle, bool owns)
-		{
-			if (handle == IntPtr.Zero) {
-				GC.SuppressFinalize (this);
-				throw new ArgumentNullException ("handle");
-			}
-			this.handle = handle;
-			if (!owns)
-				CFObject.CFRetain (handle);
-		}
-
-		~CTFont ()
-		{
-			Dispose (false);
-		}
-		
-		public void Dispose ()
-		{
-			Dispose (true);
-			GC.SuppressFinalize (this);
-		}
-
-		public IntPtr Handle {
-			get { return handle; }
-		}
-		
-		protected virtual void Dispose (bool disposing)
-		{
-			if (handle != IntPtr.Zero){
-				CFObject.CFRelease (handle);
-				handle = IntPtr.Zero;
-			}
-		}
+	public partial class CTFont : NativeObject {
 	}
 }

src/Foundation/NSArray.cs

@@ -355,6 +355,14 @@ namespace Foundation {
 			return ret;
 		}
 
+		static public T [] ArrayFromHandle<T> (IntPtr handle, Converter<IntPtr, T> creator, bool releaseHandle)
+		{
+			var rv = ArrayFromHandle<T> (handle, creator);
+			if (releaseHandle && handle == IntPtr.Zero)
+				NSObject.DangerousRelease (handle);
+			return rv;
+		}
+
 		// FIXME: before proving a real `this` indexer we need to clean the issues between
 		// NSObject and INativeObject coexistance across all the API (it can not return T)