disable Move and Rotate operations if area < 80% contained in the viewport

see #542.
Also included:
1. DRY up code for "% contained in" extent testing.
2. If action.disabled() returns a better reason, show that instead of the too_large one.

data/core.yaml

@@ -120,6 +120,7 @@ en:
         area: Moved an area.
         multiple: Moved multiple objects.
       incomplete_relation: This feature can't be moved because it hasn't been fully downloaded.
+      too_large: This can't be moved because not enough of it is currently visible.
     rotate:
       title: Rotate
       description: Rotate this object around its center point.
@@ -127,6 +128,7 @@ en:
       annotation:
         line: Rotated a line.
         area: Rotated an area.
+      too_large: This can't be rotated because not enough of it is currently visible.
     reverse:
       title: Reverse
       description: Make this line go in the opposite direction.