Improvements to Add/Edit Part interface

The Add/Edit Part interface has a number of issues, most noticeably a lack of information-hiding, unclear wording leading to error, and poor grouping. The attached patch improves this interface in the following ways:

• hides UI elements that are not relevant for the selected part type
• groups related UI elements together more clearly
• reduces the opportunities for users to make invalid selections/enter invalid data
• better layout and styling

Please see the attached screenshots.