Icon buttons!

[sarahmonster]

Ready for review, finally! It only took me over a month. 😜

There are a number of changes here, so good to take it for a spin and really test it out to get a feel for how it works in practise.

Here are the major changes:

• Added an "iconDirection" prop to allow for overwriting the default right-facing arrow. I have kinda mixed feelings about this name, since it could imply that it works for any icon and flips the icon, whereas it's only intended to be used to change the navigation button icon, for cases where your navigational direction is actually different. (So "go back" would warrant a left-facing arrow, for instance.) It's really determining the direction of the navigation action, not the icon. What about navigationDirection instead? (And have back/forward/up/down as options?)
• Added the ability to set both leading and trailing icons to any button.
• Refactored semantic and navigation buttons to use proper icons.
• Added the ability to set stackedIcon or onlyIcon, for stacked icon+text buttons, or icon-only buttons.
• Improved spacing and alignment of icons inside buttons.

Before:

After:

Things that we may want to do prior to pushing:

• Update tests to add more tests.
• Check to see that passing a null value to semantic or navigation buttons will nix the icon. (I tried this by passing <Button navigation trailingIcon={null} but I might have the syntax wrong?)
• Update README to better reflect changes. The button README is one of the first ones we wrote and thus really needs a total overhaul, so I'm kinda tempted to do it in one fell swoop after some other button improvements I had in mind. We could definitely sketch out the pertinent updates in this PR, though, and then refine later.
• I'm realising I've made the icons a good deal bigger... do we want to go with this, or should they be smaller when used as a leading or trailing icon? (stacked and onlyicon buttons definitely feel like the right size.)
• Re-animate the navigation button icons. I'll do this once we've determined the API!

Fix #216.

[tofumatt]

Oh, regarding navigationDirection… I want to make it clear that the direction is for the icon, so let's call it navigationIcon and just allow for "left" | "right" | "up" | "down" as props. I think that's fine!

Also: I like the bigger icon sizes! :shipit:

[sarahmonster]

Got distracted and decided to make our semantic buttons a bit more semantic, using shape to underscore their meaning:

[sarahmonster]

Okay! They're still not perfect, but I think a lot better. Good catch with the focus styles, there were a few things going on there that would prove to be problematic that I've patched.

We'll probably want to do a little bit of housekeeping with the interactive tokens (how did I not know these were here!?) in the future to make it more themeable and flexible, but I think that'll be cause for another, larger PR—and easier to tell what we really need once we've had a chance to work with the icon buttons a bit more!