gui2/tlabel: Add gold_small variation
theme ui, gui2/ttheme_list: New theme chooser, implement [theme] description

Adds a new 'description' attribute to [theme] and replaces the previous
instance of gui2::tsimple_item_selector used to select themes with a
brand new dialog (that's actually based on it) that allows for
displaying theme descriptions in a nice format.

Because some of the theme names aren't obvious enough to players
("UnitBox" comes to mind), we ought to provide descriptions of each
theme to save people some clicks since it's not possible at the
moment to switch the theme mid-game and see the results immediately.
theme ui: Add descriptions to the user-visible mainline themes
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