support multi-character emoji

I wonder if this might be deemed out-of-scope for an application centered around characters, but I feel one of the major use-cases for Gnome Characters is copy-pasting characters that are hard to type yourself, and nowadays emoji likely make up a major part of these.

There is a related thread at #13 focused on emoji variants, which seems to cover many common multi-character emoji, though not all of them. This leaves out some emoji such as transgender flag.

Note: Emojipedia has lists of emoji by unicode standard version, tho unfortunately it's not immediately obvious there which emoji would fall into this category.