Tabs don't follow the material design guidelines accurately.

[simeonoff]

Description

The igx-tabs styling should follow the material design guidelines more closely.

TODOs:

1. Revise min and max-widths.
2. Revise automatic color adjustment. In some instances the colors do not change accordingly.
3. Remove media queries. This is not needed and can only be more confusing for the end user.
4. The user should be able to set header shadow if they so desire via the theming schema and/or the theme mixin.