Style notebook tab buttons as ToolButtons

Part of this helps to simplify the activity notebook code. Applied in terminal as:
https://git.sugarlabs.org/terminal/mainline/commit/5d9e5f7c8869b1ef1c3c2a9db4a9f49dcf641c80

FIXME: Can't set prelight black color only for active tabs.
Replacing line 600 with ".notebook tab:active .button:prelight" doesn't work.