Add "Close Tab button" preference functionality

It implemented using own styles based on "closebuttons" attribute
of tabContainer.
Add "Browser default" option to "Close Tab button" preference

If this option set then Close Tab button will not be affected by this extension.

Introduce a new value of closeButtons preference - 10. Make it a default value.

Alias original `adjustTabstrip` function as `_originalAdjustTabstripFunc`
and override original function and call orginal aliased function in it.
It allows to add a condition (this.mCloseButtons == 10) when the original
function should be called.