Add the missing PolygonItem

Add the functionality to add labels of polygon shape. That means:

- Fix the non-working PolygonItemInserter class
- Add the missing PolygonItem class
- Add the "polygon" class to the default configuration
Use the Enter key to finish the polygon

Using abort() (aka Escape) is not the right way to finish the polygon.
Use the Enter key instead. It works this way in Inkscape. Display a
message to instruct the user to use the Enter key to finish the
polygon.
Fix the start point of the polygon
No need for _current_image_item anymore

The insertion does not happen in abort() anymore, so PolygonItemInserter
does not need to save the current image item anymore for later use.
Remove unused variable _init_pos
Allow aborting when inserting a polygon
Allow finishing a polygon using a double click
When handling keys, only pressing Return finishes the polygon
Remove default prefix
Merge pull request #82 from neingeist/feature/polygons

Add the missing polygon functionality