Screenshot popup alert

This feature adds a screenshot alert to Sugar.
On pressing <Alt>+1 key or selecting 'Take a screenshot' option from bottom toolbar
generates a popup alert where user can change the default name of that screenshot.
The popup can be dismissed by clicking 'X' button or by pressing 'Escape' key.

src/jarabe/screenshotpanel/gui.py

@@ -332,6 +332,7 @@ def _get_preview_data(screenshot_surface):
 
     preview_str = StringIO.StringIO()
     preview_surface.write_to_png(preview_str)
+
     return dbus.ByteArray(preview_str.getvalue())