Bug 927319 - Fix key presses causing reflows of the entire keyboard.

I noticed that reflows were only happening when the ```.keyboard-key.highlighted > .visual-wrapper > span:after``` selector applied ```position: absolute``` and the associated top/left attributes, when the ```highlighted``` class is applied to ```keyboard-key```.

This pull request fixes this issue by drawing the ```:after``` content before it is needed, and then setting its attribute ```opacity: 1``` when its associated key is highlighted. I have verified that this fixes the issue by enabling paint flashing.