Revert "Remove "Close Tab button" option and related functionality"

This reverts commit ec53cf3d057a8b7362ccf7a1d3504f236cb7357f.
Remove "Close Tab button on tab bar" related options
Add "Close Tab button" option functionality

It implemented using own styles based on "closebuttons" attribute
of tabContainer.

Fixes ithinc/tabutils#114.
Add "Browser default" option to "Close Tab button" preference

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