This adds an option to only detect emoji that should always present as
emoji. For example, the copyright symbol (unless followed by an emoji
presentation selector) should render as text.
Parse emoji from emoji-serenity.txt to allow displaying their names and
grouping them together in the EmojiInputDialog.
This also adds an "Unknown" value to the EmojiGroup enum. This will be
useful for emoji that aren't found in the UCD, or for when UCD downloads
are disabled.
According to TR #51, the "best definition of the full set [of emojis] is
in the emoji-test.txt file". This defines not only the emoji themselves,
but the order in which they should be displayed, and what "group" of
emojis they belong to.
