Add measurement plugin to core

[simon04]

Since we compute the area of multipolygons due to #11516, the measurement plugin could easily be integrated to JOSM core. What about the "measurement mode" – does that provide any additional features over just drawing a new way?

$ wc measurement/**/*java
  299   830 10980 measurement/src/org/openstreetmap/josm/plugins/measurement/MeasurementDialog.java
  305   886 10580 measurement/src/org/openstreetmap/josm/plugins/measurement/MeasurementLayer.java
   57   161  1869 measurement/src/org/openstreetmap/josm/plugins/measurement/MeasurementMode.java
   60   166  2327 measurement/src/org/openstreetmap/josm/plugins/measurement/MeasurementPlugin.java
  721  2043 25756 total

I also/instead/inaddition considered adding way length and area size to the "Advanced Info" of a primitive.

Any thoughts?