Add an "orientation" option to the measure tool, corresponding to
the "orientation" property of GimpToolCompass (i.e., it controls
the orientation against which the angle is measured, when not in 3-
point mode.)  The orientation is "auto" by default, so that the
angle is always <= 45 deg.  Note that the "orientation" option
affects the tool's "straighten" function, so that the layer is
rotated toward the current orientation.

Use the "pixel-angle" and "unit-angle" properies of
GimpToolCompass to read the measured angle, instead of duplicating
the angle-measurement logic, in particular, so that we benefit from
the improvements/fixes of the previous commit.