For a long time, we've had a basic set of classes for styling buttons,

called .btn. This allowed for a consistent style, with support for

"primary" (default) and "danger" (ones that invoke a destructive

operation) states. We've since built upon this, adding "segmented"

buttons (something that looks like one button but has multiple clickable

areas inside), and "split buttons" (a segmented button with a drop-down

menu of buttons).
These were a bit hacked together, and had all sorts of old junk in it

for old versions of IE, workarounds for old styling issues in Firefox,

and were just not very clean or organized. It was sort of annoying to

build upon these or modify them.
This change introduces new replacement classes for buttons (including

the variants). These use our modern component standard, consistent color

definitions, and include documentation (with ARIA attribute

requirements).
The old .btn class is replaced by .rb-c-button, which includes

modifiers for -is-primary, -is-danger, and -is-disabled.
The old .btn-segmented add-on class is replaced by

.rb-c-button-group, which contains .rb-c-button children, and can

use horizontal (default) or vertical (with -is-vertical) layouts.
The old .split-btn class is replaced by .rb-c-menu-button, which

makes use of .rb-c-button-group and .rb-c-menu. It can display a

standard menu or a button-based menu (when mixing .rb-c-button-group

into .rb-c-menu).
There are some small style differences. Namely, the button shadows and

text shadows have been removed. These weren't very noticeable, and

weren't worth ekeping around. Some of the older styling hacks for older

browsers have also been removed.
The change also tweaks the color definitions just a bit, moving some of

the lower-valued definitions for red higher-up so that we have more room

to expand the color palette. It also adds new blue and red colors, used

in the primary and danger buttons.
An upcoming change will update the JavaScript for the menu button to use

the new classes.

Made use of this in some upcoming changes. Verified the button appearance

was correct in all cases, including menu buttons and button groups.
Tested the ARIA tree when the recommended attributes were set for each

button type.
Tested the backwards-compatible classes, making sure they had the

correct styles.