Fontlist: Move cache file in a subdirectory, so as not to mess with

Cache's state tracking

Re #7510

Legacy cache has to be deleted in Cache,
*before* the first getDiskCache call.

Reword the "Clear caches" Dev menu entry & callback:
We now have an actual "Restart" action (on most, if not all, platforms).
And we clear the whole cache folder, which may host other things (e.g.,
fontlist)

frontend/apps/filemanager/filemanagermenu.lua

@@ -378,10 +378,10 @@ To:
         text = _("Developer options"),
         sub_item_table = {
             {
-                text = _("Clear readers' caches"),
+                text = _("Clear caches"),
                 callback = function()
                     UIManager:show(ConfirmBox:new{
-                        text = _("Clear cache/ and cr3cache/ ?"),
+                        text = _("Clear the cache folder?"),
                         ok_callback = function()
                             local DataStorage = require("datastorage")
                             local cachedir = DataStorage:getDataDir() .. "/cache"
@@ -389,13 +389,12 @@ To:
                                 FFIUtil.purgeDir(cachedir)
                             end
                             lfs.mkdir(cachedir)
-                            -- Also remove from Cache objet references to
-                            -- the cache files we just deleted
+                            -- Also remove from the Cache objet references to the cache files we've just deleted
                             local Cache = require("cache")
                             Cache.cached = {}
                             local InfoMessage = require("ui/widget/infomessage")
                             UIManager:show(InfoMessage:new{
-                                text = _("Caches cleared. Please exit and restart KOReader."),
+                                text = _("Caches cleared. Please restart KOReader."),
                             })
                         end,
                     })

frontend/cache.lua

@@ -39,6 +39,9 @@ end
 
 local cache_path = DataStorage:getDataDir() .. "/cache/"
 
+-- NOTE: Before 2021.04, fontlist used to squat our folder, needlessly polluting our state tracking.
+os.remove(cache_path .. "/fontinfo.dat")
+
 --[[
 -- return a snapshot of disk cached items for subsequent check
 --]]

frontend/fontlist.lua

@@ -9,6 +9,7 @@ local dbg = require("dbg")
 
 local FontList = {
     fontdir = "./fonts",
+    cachedir = DataStorage:getDataDir() .. "/cache/fontlist", -- in a subdirectory, so as not to mess w/ the Cache module.
     fontlist = {},
     fontinfo = {},
     fontnames = {},
@@ -169,12 +170,15 @@ function FontList:getFontList()
     if #self.fontlist > 0 then return self.fontlist end
 
     local cache = Persist:new{
-        path = DataStorage:getDataDir() .. "/cache/fontinfo.dat"
+        path = self.cachedir .. "/fontinfo.dat"
     }
 
     local t, err = cache:load()
     if not t then
         logger.info(cache.path, err, "initializing it")
+
+        -- Create new subdirectory
+        lfs.mkdir(self.cachedir)
     end
     self.fontinfo = t or {}