Small improvements to the stratigraphic column metadata dialog

[o-jorgensen]

Based on feedback from user testing, we can make some small improvements to the metadata dialog(s):

• [x] Hide countries that do not have any sub-options for Field and Stratigraphic column
• [x] Sort the list of countries alphabetically
• [x] Set the width of the dropdown lists to auto, so that none of the option labels are truncated with "…"

↓ Example screenshot of the current dialog