Keyboard: add Bengali Probhat layout (#8887)

Add keyboard with Bengali Probhat layout.
Bump koreader-fonts for added Noto Sans Bengali UI,
and include it among our fallback fonts.

frontend/ui/language.lua

@@ -33,7 +33,7 @@ local Language = {
         vi_VN = "Viet Nam",
         ar_AA = "عربى",
         bg_BG = "български",
-        bn = "বাঙালি",
+        bn = "বাংলা",
         el = "Ελληνικά",
         fa = "فارسی",
         ja = "日本語",