rework logic about child.VHints.activate and fix bugs
rework logic about child.VHints.activate and fix bugs

* make logic consistent
* make @setModeOpt accept a number as mode, and pass it directly
  so "execInChild" does the same thing just as child.VHints.~
* fix that @tryNestedFrame may focus child directly,
  * it's a function called by port message listeners,
    so `window.focus` will be too powerful than what we need

gdh1995/vimium-plus,gdh1995/vimium-plus,gdh1995/vimium-plus