Proposal: rename button variants to follow the spec

Material Guidelines changed the name of buttons’ variants. I’m proposing to update the variants name to follow the spec and implement those that are missing.

The rosetta stone is:

Material UI Beta 45	Current API	Material Guidelines	Proposed API
Flat Button	variant="flat"	Text Button	variant="text"
N/A	N/A	Outlined Button	variant="outlined
Raised Button	variant="raised"	Contained Button	variant="contained"
N/A	N/A	Toggle Button	toggled variant="outlined"

I’m not proposing a specific variant for Toggle Buttons because demos present them as button bars (hence could be a good idea have a specific component) where they are mostly outlined buttons with an additional toggled state, or Icon Buttons with different icons based on the state.

• I have searched the issues of this repository and believe that this is not a duplicate.

[mbrookes]

I suggest we stick with the spec names, rather than what the MCW team are using.

[eps1lon]

Legacy variant names will be deprecated with 3.2.0 i.e. deprecation warnings will be logged. The flat and raised variants were already mapped to text and contained respectively.