[2.3] GRID - Row expand/collapse icons

I've changed from png to Fontawesome icons and adjusted the vertical alignment. Also switched to a 'wireframe' style icon as I think it suits the new theme better. Thoughts?

Before:
![grid-rows-b](https://cloud.githubusercontent.com/assets/825106/3587047/a1b71010-0c35-11e4-8498-72fbe7ebf62c.png)

After:
![grid-rows-a](https://cloud.githubusercontent.com/assets/825106/3587051/a545d1b2-0c35-11e4-82cb-18785cea9d13.png)