File Manager: rename to "KOReader File Browser" and add up arrow to ".."

This should make it clearer to new users what's going on. Fixes #2529.

frontend/apps/filemanager/filemanager.lua

@@ -62,7 +62,7 @@ local function restoreScreenMode()
 end
 
 local FileManager = InputContainer:extend{
-    title = _("File Manager"),
+    title = _("KOReader File Browser"),
     root_path = lfs.currentdir(),
     -- our own size
     dimen = Geom:new{ w = 400, h = 600 },

frontend/ui/widget/filechooser.lua

@@ -154,7 +154,7 @@ function FileChooser:genItemTableFromPath(path)
     end
 
     table.sort(dirs, sorting)
-    if path ~= "/" then table.insert(dirs, 1, {name = ".."}) end
+    if path ~= "/" then table.insert(dirs, 1, {name = "⬆ .."}) end
     table.sort(files, sorting)
 
     local item_table = {}