New Tab For Android troch mortalis

Adds the New Tab button to the URL bar.
Addon settings page allows to change the new tab URL. It should be in format
"http://server.domain" like in "https://goolge.com".

'about:home', 'about:addons', and other similar URLs don't work according to the addons development documentation.

(https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/tabs/create,
see the 'url' parameter)