This parses <position> according to the following grammer:
<position> = [
[ left | center | right ] || [ top | center | bottom ]
|
[ left | center | right | <length-percentage> ]
[ top | center | bottom | <length-percentage> ]?
|
[ [ left | right ] <length-percentage> ] &&
[ [ top | bottom ] <length-percentage> ]
]
The code is a little hairy and simply tries each alternative in turn,
manually checking the possible orders. There may be a better way to
represent this.