GridField 'Add existing' action styling improvements (fixes #2567)

https://github.com/silverstripe/silverstripe-framework/issues/2567

Tested in IE8, Win/Mac Chrome, Mac FF, Mac Safari. I don't have a copy of IE9 to test on unfortunately.

Before:
![screen shot 2013-10-22 at 16 02 06](https://f.cloud.github.com/assets/1655548/1382637/2b5ba320-3b31-11e3-8055-24409d435fa1.png)

After:
![screen shot 2013-10-22 at 16 40 42](https://f.cloud.github.com/assets/1655548/1382651/3e8c9fb2-3b31-11e3-8eae-152ac05aa5f8.png)
![screen shot 2013-10-22 at 16 40 50](https://f.cloud.github.com/assets/1655548/1382655/4486fa20-3b31-11e3-9cc5-f61ce684297d.png)

After in IE8:
![screen shot 2013-10-22 at 16 41 07](https://f.cloud.github.com/assets/1655548/1382657/4a44055c-3b31-11e3-87a2-9768c321b9f2.png)
![screen shot 2013-10-22 at 16 41 15](https://f.cloud.github.com/assets/1655548/1382659/53346e86-3b31-11e3-9fed-e58a51985db1.png)