This PR fixes a pet peeve of mine: the build method(s) don't read very nicely.

First off, ggplot_build() and ggplot_gtable() are in the same file, which is messy because they are both user-facing functions, they do things independently and are also documented separately.

Secondly, I understand why ggplot_build() is hard to read: it is complicated.
However, ggplot_gtable() has a lot of gtable operation complexity, which can be easily hidden away in some choice helper functions. It becomes a lot simpler to understand what ggplot_gtable() is doing if the steps are separate functions.