stack: More readability when adding transitions

stack: More readability when adding transitions

This small refactor makes the code more readable when adding more
transition types that have left, right, up, and down variations.

It adds inline boolean functions to tell information about transition
types (avoiding long if clauses) and changes long chains of "else if
(transition_type == ...)" into switch statements. Both are only likely
to get longer as more transition types are added.

https://bugzilla.gnome.org/show_bug.cgi?id=726676