At first you could think that this refactor would not be
necessary, because `ACHANNEL_FIRST` exists already.
It contained the small y offset that all channels had.
Unfortunately, a lot of code assumed that
`ACHANNEL_FIRST = -ACHANNEL_HEIGHT`, making the
define pretty much useless. This refactor fixes that
for the action and nla editor.
As a nice side effect, this patch fixes channel box select.
Before there was always have a half-channel offset.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D4783
fa59346c13