Make tabs more visible

[jiajasper]

The current tabs design is not obvious to users, they didn't notice that files were opened in tabs - they all thought it was dynamically loaded in one single viewport.

Hence, we want to make the tabs use the full 32px height (align with the logo size which is 32 by 32) and make the scroll bar appear beneath the tabs (instead of sharing the same space with the tabs).

We also would like to make the font bigger and bolder to make them more visible.

Figma link: https://www.figma.com/file/tgjz0PLgrPoJ9gh6nqkNGw/PlayCanvas-Texture-Viewer?node-id=9%3A252