Make Imagery offset correction options default or at least more prominent

[RicoZ]

The offset database with the plugin (​http://wiki.openstreetmap.org/wiki/Imagery_Offset_Database) are very helpful to reduce systematic errors introduced by skewed aerial imagery (most notably Bing).

Even as an halfway experienced contributor I do occasionally forget to activate it, there was also a recent discussion on the talk-de mailing list about the possibilities.

I could imagine following approach: when activating Imagery/Bing, present the user with the following dialog:

• download imagery offset from database
• correct offset manually
• ignore for now
• ignore allways
• further informations - ​http://wiki.openstreetmap.org/wiki/Imagery_Offset_Database