Make parts lists more ergonomic

[k3DW]

Closes #25

• Batch editing works as one would expect
• If all the selected parts have the same value for a specific widget, that value is shown, otherwise the widget value is left empty
• The part displayed in the viewport is whichever of the parts was selected first of the currently selected batch

Closes #27

• This only applies if a part has a "base quantity", i.e. a default quantity parsed from the file name
• If the quantity is higher than the base quantity, an up arrow "↑" is displayed
• If the quantity is lower than the base quantity, a down arrow "↓" is displayed