Nit: fix code style.
Work around the limitation of windows API when setting thumbnail toolbar buttons.

- New API: InsertThumbBarButton, AppendThumbBarButton, RemoveThumbBarButton.
- Though MSDN said that "Buttons cannot be added or deleted later, so this must
  be the full defined set. Buttons also cannot be reordered.", we can work
  around it by: when first time adding button, initialize all of the possible
  seven buttons and hide them, except the button adding. In the next time adding
  button, just show it, which can make it looks like it is added on the fly.