Improve plugin dialog UX

[KoBeWi]

Partially supersedes #36677

• Subfolder and script name are now optional
  • If empty, the default subfolder path will be the plugin name converted to snake_case
  • If empty, the script name will be the same as subfolder
  • Script name extension is optional and will be added based on the selected language
• Aligned labels to the right
• Removed "Update" button. The list is updated on dialog focus, so manual refresh is useless
• Changed "Create" button to "Create New Plugin"
• Misc code cleanup
  

[akien-mga]

Thanks!