ENH: Change inplace-add to normal add to support alternate syntax

Widgets can be added to Plugins inline, and Plugins can be added inline to Viewers.

skimage/viewer/plugins/base.py

@@ -110,7 +110,7 @@ class Plugin(QtGui.QDialog):
         self.layout.addWidget(widget, self.row, 0)
         self.row += 1
 
-    def __iadd__(self, widget):
+    def __add__(self, widget):
         self.add_widget(widget)
         return self
 

skimage/viewer/viewers/core.py

@@ -96,7 +96,7 @@ class ImageViewer(QtGui.QMainWindow):
 
         self.connect_event('motion_notify_event', self.update_status_bar)
 
-    def __iadd__(self, plugin):
+    def __add__(self, plugin):
         plugin.attach(self)
         return self