@gib: Update text editing UI

This does some small UI updates to the text editing toolbar:

![image](https://cloud.githubusercontent.com/assets/555859/4910619/ef4ef8de-6481-11e4-99b8-955431f14add.png)

![image](https://cloud.githubusercontent.com/assets/555859/4910569/6cdaff42-6481-11e4-8fa6-b2d8de9ba25c.png)

~~Also Scribe (The Guardian's composable WYSIWYG text editor) [doesn't have a "destroy" method](https://github.com/guardian/scribe/issues/95). Making it difficult to incorporate with thick-client apps like this. So [I'm in the middle of a PR](https://github.com/guardian/scribe/pull/288) to add that destroy functionality. In the meantime I'm pointing to my fork to keep Positron moving.~~