Restyle elevation 5 as 12dp; add elevation 6

[keanulee]

Required to update the raised elevation for paper-fab. See PolymerElements/paper-fab#35

Technically this is a breaking change for anyone who uses elevation="5" (it is now a 12dp shadow instead of 16dp), but AFAIK none of our elements rely on this, and this purely cosmetic change will have little impact where it is used.

[addyosmani]

Change LGTM. Per the comment in PolymerElements/paper-fab#35 (comment) looks like we're fine adding the docs after this lands. This is a breaking change so will we want to reflect that in the versioning? 🐑

[keanulee]

@addyosmani While this is technically a breaking change, I think we want to hold off on a major version bump - simply updating the styling doesn't feel like a major breaking change.

As for paper-fab, it's styling won't actually be corrected until after this PR lands and I can update its calculateElevation method. That also doesn't feel like a major change, since this is essentially "fixing" its styles.

[notwaldorf]

Breaking the style in general, out of context, is definitely a breaking change, friends, whether it breaks our elements or anybody who uses this element. The argument can be made, however, that in this case we've just implemented the wrong height for this elevation. I tried to see how many people used elevation 5, but the GitHub text search is garbage and I got 75k results of not actually elevation="5".

The change LGTM, but pinging @cdata to keep us honest about the version.

[keanulee]

ping @cdata

[btelles]

ping @cdata

[notwaldorf]

@keanulee I'm going to remove myself from this so that I can clean up my incoming PR queue :)