Embedded fonts toggle: disabled if no embedded font

Have the enabled/disabled state of the toggle show the
presence or not of embedded fonts in the current book.
Also show the names of the embedded fonts in the
help_text InfoMessage.

frontend/document/credocument.lua

@@ -797,6 +797,10 @@ function CreDocument:getFontFace()
     return self._document:getFontFace()
 end
 
+function CreDocument:getEmbeddedFontList()
+    return self._document:getEmbeddedFontList()
+end
+
 function CreDocument:getCurrentPos()
     return self._document:getCurrentPos()
 end

frontend/ui/data/creoptions.lua

@@ -1,9 +1,10 @@
 local Device = require("device")
 local Screen = Device.screen
+local ffiUtil = require("ffi/util")
 local optionsutil = require("ui/data/optionsutil")
 local _ = require("gettext")
 local C_ = _.pgettext
-local T = require("ffi/util").template
+local T = ffiUtil.template
 
 -- Get font size numbers as a table of strings
 local tableOfNumbersToTableOfStrings = function(numbers)
@@ -700,12 +701,24 @@ Whether enabled or disabled, KOReader's own status bar at the bottom of the scre
                 args = {false, true},
                 default_arg = nil,
                 event = "ToggleEmbeddedFonts",
-                enabled_func = function(configurable)
+                enabled_func = function(configurable, document)
                     return optionsutil.enableIfEquals(configurable, "embedded_css", 1)
+                            and next(document:getEmbeddedFontList()) ~= nil
                 end,
                 name_text_hold_callback = optionsutil.showValues,
                 help_text = _([[Enable or disable the use of the fonts embedded in the book.
 (Disabling the fonts specified in the publisher stylesheets can also be achieved via Style Tweaks in the main menu.)]]),
+                help_text_func = function(configurable, document)
+                    local font_list = document:getEmbeddedFontList()
+                    if next(font_list) then
+                        local font_details = {}
+                        table.insert(font_details, _("Embedded fonts provided by the current book:"))
+                        for name in ffiUtil.orderedPairs(font_list) do
+                            table.insert(font_details, name .. (font_list[name] and "" or T("  (%1)", _("not used"))))
+                        end
+                        return table.concat(font_details, "\n")
+                    end
+                end,
             },
             {
                 name = "smooth_scaling",

frontend/ui/data/optionsutil.lua

@@ -101,7 +101,10 @@ function optionsutil.showValues(configurable, option, prefix, document)
     end
     if option.help_text_func then
         -- Allow for concatenating a dynamic help_text_func to a static help_text
-        help_text = T("%1\n%2\n", help_text, option.help_text_func(configurable, document))
+        local more_text = option.help_text_func(configurable, document)
+        if more_text and more_text ~= "" then
+            help_text = T("%1\n%2\n", help_text, more_text)
+        end
     end
     local text
     local name_text = option.name_text_func