Changing the template should ask for a reload

[sabberworm]

Currently, changing the template name reloads the edit area with the new content template if the newsletter has not been saved and does nothing otherwise (except, of course, using another body and css template when sending). The behaviour should actually be a bit more sophisticated:

If no changes have ever been made to the edit area, the template should just be reloaded regardless. Otherwise, a dialog should pop up, informing the user of the consequences (loss of previously entered data).

This behaviour should be the same, regardless of whether the newsletter has been saved before or not (this might need a new flag in the database).