notebook: Ignore and deprecate style properties

notebook: Ignore and deprecate style properties

Deprecate initial-gap, tab-curvature and tab-overlap properties. All
their features can be achieved using CSS.

This CSS achieves the same effect as a 10px initial gap:
  notebook header {
    margin-left: 10px;
    margin-right: 10px;
  }

A tab overlap of 10px can be achieved via:
  notebook tabs {
    margin-left: 10px;
  }
  notebook tab {
    margin-left: -10px;
  }

And tab curvature is essentially the same as padding or border on a tab.