[Feature] Skin engine Themes grab text from a file %ft(file, line)

allow the skin engine to read text files and return a particular line

you then can use ss on that string to allow display of strings from the file
(Playername comes to mind)

able to be used as conditional
%?ft(filename)<Found|Not Found>

if (selected) line of file is empty the tag is treated as #COMMENT

bugfix:
%t(n)%?x<text|text>
would ignore the specified timeout defaulting to 2 seconds

bugfix: cabbiev2.128x160x16.wps was missing %Sx()
for translation on 'Next Track:'

playername.txt generated at boot if it doesn't exist contents: 'Rockbox!'

Change-Id: I04ea4fd411f74c7c6e672657949aa520c2f86f95

utils/skinupdater/tag_table.c

@@ -70,6 +70,7 @@ struct tag_info legal_tags[] =
     { "fn", "" },
     { "fp", "" },
     { "fs", "" },
+    { "ft", "" },
     { "fv", "" },
     { "d"  , "I" },