Finish clipboard implementation

* uses autoreleasepool
* implments Clear method
* fixes SetText when text is null

src/Avalonia.Native.OSX/clipboard.mm

@@ -5,16 +5,26 @@ class Clipboard : public ComSingleObject<IAvnClipboard, &IID_IAvnClipboard>
 public:
     virtual HRESULT GetText (void** retOut)
     {
-        NSString *str = [[NSPasteboard generalPasteboard] stringForType:NSPasteboardTypeString];
-        *retOut = (void *)str.UTF8String;
+        @autoreleasepool {
+            NSString *str = [[NSPasteboard generalPasteboard] stringForType:NSPasteboardTypeString];
+            *retOut = (void *)str.UTF8String;
+        }
         return S_OK;
     }
     
     virtual HRESULT SetText (char* text)
     {
-        NSPasteboard *pasteBoard = [NSPasteboard generalPasteboard];
-        [pasteBoard clearContents];
-        [pasteBoard setString:@(text) forType:NSPasteboardTypeString];
+        @autoreleasepool {
+            NSPasteboard *pasteBoard = [NSPasteboard generalPasteboard];
+            [pasteBoard clearContents];
+            [pasteBoard setString:@(text) forType:NSPasteboardTypeString];
+        }
+        return S_OK;
+    }
+
+    virtual HRESULT Clear()
+    {
+        [[NSPasteboard generalPasteboard] clearContents];
         return S_OK;
     }
 };

src/Avalonia.Native/Stubs.cs

@@ -21,17 +21,26 @@ namespace Avalonia.Native
 
         public Task ClearAsync()
         {
+            _native.Clear();
             return Task.CompletedTask;
         }
 
         public Task<string> GetTextAsync()
         {
             var outPtr = _native.GetText();
+<<<<<<< Updated upstream
             return Task.FromResult(Marshal.PtrToStringAnsi(outPtr));
+=======
+            var text = Marshal.PtrToStringAnsi(outPtr);
+            return Task.FromResult(text);
+>>>>>>> Stashed changes
         }
 
         public Task SetTextAsync(string text)
         {
+            _native.Clear();
+            if(text != null)
+                _native.SetText(text);
             return Task.CompletedTask;
         }
     }

src/headers/avalonia-native.h

@@ -217,6 +217,7 @@ AVNCOM(IAvnClipboard, 0f) : virtual IUnknown
 {
     virtual HRESULT GetText (void** retOut) = 0;
     virtual HRESULT SetText (char* text) = 0;
+    virtual HRESULT Clear() = 0;
 };
 
 extern "C" IAvaloniaNativeFactory* CreateAvaloniaNative();