Fixes #83: Fixes the selection bar movement in RTL.

[bicknellr]

The current implementation assumes that changing selection from a higher index tab to a lower index tab implies moving left on the screen (and lower to higher, right).

[cdata]

Ah, such a fun hack. I wonder if we could measure a property that has nothing to do with layout, and thus avoid potential layout recalculations (however small they might be). WDYT?

Also, @notwaldorf you should check this out, since it is a little on the crazy (crazy awesome?) side ;)

[bicknellr]

Just as a heads up, I might be submitting a PR to add an RTL check in iron-menubar-behavior soonish and make use of it here instead. That one's going to use getComputedStyle since it only needs to happen once per keystroke and [dir=rtl] and friends would leave a few holes (:dir(rtl) isn't supported anywhere yet :( ).

[cdata]

Holding off on reviewing this until pending iron-menu-behavior PR shows up.

[bicknellr]

self ping!

[bicknellr]

(@cdata, PolymerElements/iron-menu-behavior#41 is the PR this is waiting on.)

[cdata]

Noting that this PR is transitively blocked on Polymer/polymer#2717

[bicknellr]

@cdata Just rebased as all the blockers are in. I'm not sure how to test this though since it's an animation..

[cdata]

LGTM 👍