Text icons are in GameInfo/CIV5IconFontMapping.xml. There's about 80 of them. The first few are used heavily (ICON_GOLD, ICON_PRODUCTION, etc.) but the later ones like flower, pirate, omega, etc. are also pretty useful for new resources and such, which is what I use them for in my mod. Most of the names are pretty self-explanatory, and it's easy enough to see what they all do for yourself.
Note that these are only for Text icons, as in, the little hammer/citizen/whatever icons you see in the Civilopedia or help texts. Anything bigger than that (like tech icons) will be in an Icon Atlas, and those files just aren't available without some file unpacking.
As to the question in your title, the TXT_KEY_* entries are all in NewText/en_US/CIV5GameTextInfos_*.xml (assuming you're using American English; replace the en_US with the appropriate directory for you). The one you probably want is the _Civilizations.xml, but it might be in _Objects, _Jon, or the massive _Civilopedia.xml file. (I don't have the files in front of me.)