Save improvement

[hspitzer]

Some bugfixes / improvements for saving:

• fixes accidental overwriting of annotations of slices that are currently not loaded
• enables saving of annotations on previously empty slices
• show alert/info box when saving - as confirmation for the user
• added flag in configuration.json that can toggle between saving multiple slices at one time vs. just saving the current slice (the users at the INM would rather just save the current slice, because this gives more control over what gets saved in the database)

also temprorarily fixes the issue of the multiple annotations being loaded when navigating with the slider (see #31) by disabling the loading of any images until the slider has stopped moving. For IE this issue might still appear.

makes polygon drawing undoable.