add generic plugin to copy labels from the previous image/frame (fixes #34)

sloth/plugins/__init__.py

@@ -0,0 +1,22 @@
+from PyQt4.QtGui import *
+from PyQt4.QtCore import *
+
+class CopyAnnotationsPlugin(QObject):
+    def __init__(self, labeltool):
+        QObject.__init__(self)
+        self._labeltool = labeltool
+        self._wnd = labeltool.mainWindow()
+        self._sc  = QAction("Copy labels from previous image/frame", self._wnd)
+        self._sc.triggered.connect(self.copy)
+
+    def copy(self):
+        current = self._labeltool.currentImage()
+        if current is not None:
+            prev = current.getPreviousSibling()
+            if prev is not None:
+                for annotation in prev.children():
+                    copied = dict(annotation.iteritems())
+                    current.addAnnotation(copied)
+
+    def action(self):
+        return self._sc