specify |window| for own extended property of global scope
fix missing id of menuitem
optimize command string
remove setting anchor of PanelUI menu

* I have usually hidden the PanelUI menu button and popped the menu up to the
  URL bar. But the hidden menu button seems to be incompatible with Fx native
  behavior. So I always show the menu button now.
compact code
relocate buttons
disable unused clear button
optimize code
minor fix

* update comments