[feat] SkimToWidget chapter markers & next/prev chapter/bookmark (#3389)

https://github.com/koreader/koreader/issues/2819#issuecomment-332261968
+ refactoring code in skimtowidget
+ add chapter markers
+ add next/prev chapter buttons
+ add next/prev bookmark buttons

frontend/ui/widget/button.lua

@@ -107,9 +107,9 @@ function Button:init()
     end
 end
 
-function Button:setText(text)
+function Button:setText(text, width)
     self.text = text
-    self.width = nil
+    self.width = width
     self:init()
 end