Add an offset_angle parameter to gimp_constrain_line(), which
offsets the radial lines by a given angle.

Add gimpdisplayshell-utils.[ch], with two new functions:

  - gimp_display_shell_get_constrained_line_offset_angle():
    Returns the offset angle to be passed to
    gimp_constrain_line(), in order to constrain line angles in
    display space, according to the shell's rotation angle and
    flip mode.

  - gimp_display_shell_constrain_line():  A convenience function
    which calls gimp_constrain_line() with the said offset angle.

Use the new functions in all instances where we constrain line
angles, so that angles are constrained in display space, rather
than image space.

The only exception is GimpEditSelectionTool, which keeps
constraining angles in image space, since it's not entirely obvious
that we want to constrain angles of dragged layers/selections in
display space.