Add an "orientation" property to GimpToolCompass, which can be one
of "auto", "horizontal", or "vertical", and which controls the
orientation of the line against which the angle is measured, when
not in 3-point mode (previously, the line would always be
horizontal.)  When "orientation" is "auto", the orientation is
automatically set to either horizontal or vertical, such that the
measured angle is <= 45 deg.

Keep the line horizontal, or vertical, in display-space, rather
than in image-space, so that the compass works correctly even when
the canvas is rotated and/or flipped.

Fix the compass's behavior when the image's horizontal and vertical
resolutions are different, both with and without dot-for-dot.

Add "pixel-angle" and "unit-angle" read-only properties, which
return the measured angle either with or without taking the image's
resolution into account, respectively.  These properties will be
used by the measure tool in the next commit, instead of having it
implement its own angle calculation.