Add configuration to hide or show New Tab button

[kaizoku]

This adds a new configuration option to either show or hide the New Tab button, based on the configuration for the All Tabs button.
Also adds a line in the default cascade-config.css to show people how to configure it, same as the All Tabs button.

[unseen-ninja]

It seems like this change introduces some padding issues.. :/

I personally just hide the New-Tab button by removing it from the tabbar via the »Customise Toolbar«-Option as this doesn't require any additional CSS but yields the same result. Tho there may be a reason to instead use your proposed solution; if so please let me know about it. c: