Sprite Sheet Editor - Remove "Export", "Color Swap", and "Import" Buttons

[RobDavenport]

Since these functions will exist in a future version, we should remove these buttons temporarily until we can properly add them in.

The buttons that need to be removed are:

• Export , from the Sprite Sheet Editor
• Color Swap, from the Sprite Sheet Editor
• Import, from the Sprite Sheet List

[RobDavenport]

Note that "Import" for Sprite Sheet List is indended for this feature: #63