Hyphenation: add custom hyphenation rules (#7746)

The hyphenation of a word can be changed from its default
by long pressing for 3 seconds and selecting 'Hyphenate'.
These overrides are stored in a per-language file, i.e:
koreader/settings/user-German.hyph.

frontend/ui/widget/frontlightwidget.lua

@@ -583,6 +583,7 @@ function FrontLightWidget:onCloseWidget()
     UIManager:setDirty(nil, function()
         return "flashui", self.light_frame.dimen
     end)
+    return true
 end
 
 function FrontLightWidget:onShow()