Create a Swing window that shows mod loading progress at startup

[EmotionalLove]

This pull request creates a progress window that appears before Minecraft starts that notifies the user of the task that the Mod Loader is working on. The progress window only opens on instances of the Minecraft client, and automatically closes when the mod loader is finished loading mods.

Please see the attached screenshot that shows it in action.

[Robijnvogel]

Looks cool! :D

[shedaniel]

Why not use the default look and feel on the os? :p

[EmotionalLove]

That might be a good idea.