Fix regression: bug copying & pasting regular cels in the timeline

Before this fix if an image were allocated in the clipboard, then you
copied a Cel and wanted to paste it to other Cel, the previous image
were pasted in the canvas instead the last Cel copied in the timeline.

How to reproduce the bug:

- Make a sprite.
- Draw something on the canvas.
- Select a drawn region and copy the selection (the image is now in
  the clipboard).
- Add a frame in the time line.
- Copy the Cel of frame 1.
- Select the empty Cel of the frame 2.
- Paste: you'll see the image selected in step 3 was pasted on the
  canvas instead the last Cel copied on the timeline.

src/app/util/clipboard.cpp

@@ -341,6 +341,8 @@ void Clipboard::clearMaskFromCels(Tx& tx,
 
 void Clipboard::clearContent()
 {
+  if (use_native_clipboard())
+    clearNativeContent();
   m_data->clear();
 }
 

src/app/util/clipboard.h

@@ -96,6 +96,7 @@ namespace app {
     bool copyFromDocument(const Site& site, bool merged = false);
 
     // Native clipboard
+    void clearNativeContent();
     void registerNativeFormats();
     bool hasNativeBitmap() const;
     bool setNativeBitmap(const doc::Image* image,

src/app/util/clipboard_native.cpp

@@ -54,6 +54,12 @@ namespace {
 
 }
 
+void Clipboard::clearNativeContent()
+{
+  clip::lock l(native_window_handle());
+  l.clear();
+}
+
 void Clipboard::registerNativeFormats()
 {
   clip::set_error_handler(custom_error_handler);