Several positions do not work with `orientation = "y"`

[aphalo]

Unit tests need to build plots to test that this works. Although tests' code coverage is > 90% the positions are mostly tested by building Position objects without testing that they work correctly together with 'ggplot2', i.e., supporting all the expected functionality when a "gg" object is rendered. See also #58.

• [ ] Add unit tests that build plots both normally and flipped.
• [ ] Add the missing support to the positions that do not work correctly with orientation = "y".
• [ ] Add some examples using orientation = "y" to the functions' documentation.
• [ ] Add explanation and examples using orientation = "y" to the vignette.