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

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

scroll timeouts now persist thru trackchange

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

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

Change-Id: I961910e01be052ef902f77e6d92fc3e367ffe9d0

lib/skin_parser/tag_table.c

@@ -50,6 +50,7 @@ static const struct tag_info legal_tags[] =
     TAG(SKIN_TOKEN_FILE_SIZE,             "fs", "", SKIN_REFRESH_STATIC),
     TAG(SKIN_TOKEN_FILE_VBR,              "fv", "", SKIN_REFRESH_STATIC),
     TAG(SKIN_TOKEN_FILE_DIRECTORY,        "d"  , "I", SKIN_REFRESH_STATIC),
+    TAG(SKIN_TOKEN_FILE_TEXT,             "ft" , "F|i", SKIN_REFRESH_STATIC),
 
     TAG(SKIN_TOKEN_FILE_BITRATE,          "Fb", "", SKIN_REFRESH_STATIC),
     TAG(SKIN_TOKEN_FILE_CODEC,            "Fc", "", SKIN_REFRESH_STATIC),

lib/skin_parser/tag_table.h

@@ -155,6 +155,7 @@ enum skin_token_type {
     SKIN_TOKEN_FILE_SIZE,
     SKIN_TOKEN_FILE_VBR,
     SKIN_TOKEN_FILE_DIRECTORY,
+    SKIN_TOKEN_FILE_TEXT,
 
     /* Image */
     SKIN_TOKEN_IMAGE_BACKDROP,