minor UI tweaks

AddSubCategoryPage - Separator is now replaced with a border (also added
style for border)

Some Comic-related resource strings now read 'Comic Series' instead of
'Comic' or 'Comics'.

Added 'Item Field Label' style for use throughout Item Pages.

ExportPage - Redid page layout (it's still just one button)
Collection pages now truncate multiline notes

Added new helper property to Item: NotesSummary (might be temporary)