Remove highlightElevation from RawZefyrButton

[kolja-esders]

Issue

Currently, there is a pretty intense elevation of active toolbar buttons during a long-press. The elevation is also visible for a short period of time when the user toggles an active button to become inactive.

This behavior does not look intended and does not seem coherent with the perception of a simple on/off toggle button (suddenly there is a third dimension out of nowhere).

Fix

The PR explicitly sets the highlightElevation of the underlying RawMaterialButton to 0.

[codecov]

Codecov Report

Merging #103 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #103   +/-   ##
=======================================
  Coverage   92.11%   92.11%           
=======================================
  Files          12       12           
  Lines        1090     1090           
=======================================
  Hits         1004     1004           
  Misses         86       86

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