QtFRED Asteroid Dialog

[MjnMixael]

Entirely rebuilds QtFRED's asteroid dialog to match the new features and fix bugs. Some notable changes here worth mentioning and I'll explain in more detail on Discord:

• Utilize Qt's automatic slot/signal connection system in place of tons of manual connect() calls in the initializer
• Make sure the abstract dialog's apply() returns its value instead of always returning true
• Remove signals from the .ui file as those are a touch opaque for anyone not familiar with Qt and instead handle it with the slot/signal system
• Rearrange how OK/Cancel, ESC, Close are intercepted to properly handle all cases especially including model validation on Apply after Reject -> Save Changes

This was also a good opportunity to add a constructor to the asteroid_field struct for easy resetting.

Fixes #1687