Plus button tab icon and minimized drawer

[Gavin Xu]

Hi devtools-devs,

I would like to propose some UI changes to the DevTools window, including plus button enhancement, tab icons and minimized drawers in this design document: https://docs.google.com/document/d/1s21w-S2A4tWmBehug525g-tfRQ5HVjfS1D8C4YeTzXs/edit?usp=sharing 

Here is a short URL for the doc: http://bit.ly/47ImPBX

Please take a look and share your feedback when you get a chance.

Thanks,

Guangyue Xu