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.
By fiddling, I meant more that I wanted to change other settings on the text elements--for instance make them bold, italics, colored, etc.--but let it get the same default values otherwise.

The move to the top level here was because I had places I wanted to use the same text sizes in slides but didn't make sense with the centered helpers. It was annoying to have to reference `Wingtips::Slide::ENORMOUS_SIZE` in my slide definitions, hence the move.

This was probably the most subjective of the PR's I logged, so happy to hear what you think about it further.