[2.3] GRID - Header bar design

This PR and https://github.com/modxcms/revolution/pull/11672 results in the following design tweaks to the grid header bar and sort options button. Comments welcome!

Before:
![header-bar-b](https://cloud.githubusercontent.com/assets/825106/3555900/d2ee6a78-091c-11e4-8dd1-bf7b3e8411bf.png)

After:
![header-bar-a](https://cloud.githubusercontent.com/assets/825106/3555902/d70892d2-091c-11e4-96a9-f82c1ec41e5c.png)