Changed StampBrush to do rectangle-erase on right-click

Feature requested by elvisish, but can't be merged like this. Instead,
the mouse interaction in general would need to be made more
configurable.

This change currently leaves no way to capture a stamp from the map.

src/tiled/stampbrush.cpp

@@ -317,7 +317,7 @@ void StampBrush::beginCapture()
     mBrushState = BrushState::Capture;
     mCaptureStampHelper.beginCapture(tilePosition());
 
-    setStamp(TileStamp());
+    updatePreview();
 }
 
 void StampBrush::endCapture()
@@ -327,12 +327,12 @@ void StampBrush::endCapture()
 
     mBrushState = BrushState::Free;
 
-    const bool cut = mModifiers & Qt::ShiftModifier;
-    TileStamp stamp = mCaptureStampHelper.endCapture(*mapDocument(), tilePosition(), cut);
-    if (!stamp.isEmpty())
-        emit stampChanged(stamp);
-    else
-        updatePreview();
+    const bool allLayers = false;
+    const bool mergeable = false;
+    mapDocument()->eraseTileLayers(mCaptureStampHelper.capturedArea(tilePosition()),
+                                   allLayers, mergeable);
+
+    updatePreview();
 }
 
 /**