Revised methods for title slides

Found we only had methods for some sizes, and didn't have a consistent way to
fiddle the formatting when you wanted *almost* that slide with a little
difference.

Also has a hacky vertical alignment that works ok for single lines of text.
Worth improving later, but gets it done for now.
I think putting them under `Wingtips` is a reasonable compromise between being overly verbose and being safe about our constants. Will make that move before carry on with a merge on this.