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" 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.
Merge branch 'dev-closebuttons' into dev

Implement "Close Tab button" preference functionality from scratch.

It doesn't contain "tab bar close button" related options.

Also it contains a new option "Browser default" which makes Close Tab
buttons behave in a browser default way and not being affected by
this extension.

Fixes ithinc/tabutils#114.
Update README for a future release