Sparser z-index values

[gkalpak]

Currently (v0.6.0-rc3-master-98c3152), the values used for the z-index property of various components, range from 0 to 10 (with the exception of md-tab md-tab-label at 100, which is likely a bug anyway).

Since z-index defines the relative positions of elements in the z axis, this makes it difficult (read "impossible") for users to insert their own elements "in between".

Searching through the spec, I couldn't find any mention of those particular z levels (1 - 10) or of a suggested limit in the number of z levels. (Do let me know, if I have missed it though.)

So, I think it would greatly increase flexibility, if the z-indices were distributed over a larger range.