fixes incorrect position computation in the case where right and center are given
<right> and <bottom> should measure from the right and bottom respectively
drop <centerx> and <centery> and use <centerleft|right|top|bottom> where right and bottom measure from the right and bottom
[confluence] update positioning so that <right> is measured from the right
Merge pull request #3669 from jmarshallnz/right_means_right

Control positioning: Make <right>, <bottom> measure from right and bottom