Rework toolbar design

[julienamsellem]

Purpose of this PR

Jira
https://jira.unity3d.com/browse/GVFXG-143
Design docs
https://docs.google.com/document/d/1oG7-tKzlEDqQCRnw0KVsRqldVUyTY5gBfVuF-5tPx2Q/edit
https://docs.google.com/document/d/1VS4LKjZmfx40kutvb39h3sIL6UCSM9m92IxuluxGTeI/edit#

Before

After

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Testing status

Tested on dark and light themes with 100% and 150% DPI screens

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Comments to reviewers

Known bug: when DPI is different than 100% the drop down popups can be position at a "not ideal" position.
The bug is most probably on UIToolkit side. I warned the #dev-uitoolkit channel about it

[github-actions]

Hi! This comment will help you figure out which jobs to run before merging your PR. The suggestions are dynamic based on what files you have changed.
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