Move Import Library.. to file menu, Minor fixes in FunctionNamePrompt

### Fixes

The Libraries menu is brought too high up in the interface, even though I reckon it is a fairly seldomly used feature.  It confuses users with Packages, too.  I've moved it into the file menu where it is a bit less confusing for first time users.

The `FunctionNamePrompt` had some minor issues.  The gradient looked odd and the placeholder text was incorrectly placed and of the wrong font size.  It's fixed now.  The window also did not come up centered on the main Dynamo window.  This is annoying and now fixed.  BTW, I fixed all of the other windows to  come up centered in another recent pull request so the About box and package manager windows should also be correctly centered.

### Screenshots

![centered](https://cloud.githubusercontent.com/assets/916345/4923691/c1933ef0-651c-11e4-9558-d8f3cb7b2186.PNG)

![importlibrary](https://cloud.githubusercontent.com/assets/916345/4923692/c19599a2-651c-11e4-9aad-fac2642ff03b.PNG)


### Reviewers

* [x] @ikeough