Prevent page elements from displaying on top of highlights

Use position: relative; z-index: 0; on .ve-ce-surface to create a new
stacking context [1], which prevents elements with z-index from being
positioned "higher" than .ve-ce-surface itself.

As .ve-ui-overlay has a z-index of at least 1 (possibly more in case
of some MW skins), elements that are part of .ve-ce-surface will no
longer be displayed atop overlay highlights, regardless of how high
their z-index is.

[1] https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Understanding_z_index/The_stacking_context

Bug: 70074
Change-Id: I6c312269998ef775bc852d391cd1d644a0566531
Merge "Prevent page elements from displaying on top of highlights"