ReaderFooter: Add a compact prefix for frontlight_warmth
ReaderFooter: Add a compact prefix for frontlight_warmth

Fix #8398

Also, tweak the actual codepoints being used so that they somewhat look
like their icon brothers. May or may not actually be compact :D.

koreader/koreader,NiLuJe/koreader,koreader/koreader,NiLuJe/koreader,Frenzie/koreader,poire-z/koreader,Frenzie/koreader,poire-z/koreader