Improves WPAlertView UITextField behavior.

Improvements affecting the link helper in the post editor and the custom image size dialog:
Make the keyboard for text show a "Next" button, tapping/pressing enter should jump to URL
Make the keyboard for URL show a "Done" button, tapping/pressing enter should insert the link

Affecting the link helper:
Select URL field by default if the link helper is launched with selected text

Fixes #367