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

This fixes a bug where only `<right>` and `<centerx>` were given.

In addition, we change so that `<right>` and `<bottom>` are measure from right and bottom respectively.  This makes some positioning easier (I want it 10 pix from the right edge of the screen/parent) and some slightly harder (I want to position a right aligned label so that the right edge is 100px from the left).

@ronie: Unfortunately fixing this in confluence is non-trivial.  I could do a hack, where I change all the existing `<right>` measurements to use 56r for example - the r would be interpreted as measured from the LEFT hand side in this instance (i.e. it doesn't really make sense, but support is left in for back compat).  Ofcourse, some of the `<right>` stuff already uses r positioning, so these should just drop the r.

Instead of fixing these automatically in this way, and having code that is unintuitive, I thought it was better to mark them as clearly needing fixing, and then give you the opportunity to fix in whichever manner you deem most appropriate.  Thus, 52bda4a does this.  Let me know if you'd prefer me to mark them in some other way.