Merge image_style_add_form() into image_style_form()

Problem

• image_style_add_form() is a completely separate form, duplicating the entire code of image_style_form().

Goal

• Apply common patterns.

Proposed solution

1. Remove image_style_add_form().
2. Change the menu/router item to point to a new image_style_add_page() page callback function, which creates an empty $style thingie and passes that to image_style_form().
3. Ensure that page titles are correct.

Related issues

• #606598: Human readable image-style names
• #1668820: Concept, base class, and interface for Configurables (e.g., node types, image styles, vocabularies, views, etc)