Add ability to show "New Private Tab" button after "New Tab" button

[Infocatcher]

Requested here: https://addons.mozilla.org/addon/private-tab/reviews/495488/

We can use additional button (just like for built-in "New Tab" button):

and tricks like
chrome://browser/content/browser.css

#tabbrowser-tabs:not([overflow="true"]) ~ #alltabs-button,
#tabbrowser-tabs:not([overflow="true"]) + #new-tab-button,
#tabbrowser-tabs[overflow="true"] > .tabbrowser-arrowscrollbox > .tabs-newtab-button,
#TabsToolbar[currentset]:not([currentset*="tabbrowser-tabs,new-tab-button"]) > #tabbrowser-tabs > .tabbrowser-arrowscrollbox > .tabs-newtab-button,
#TabsToolbar[customizing="true"] > #tabbrowser-tabs > .tabbrowser-arrowscrollbox > .tabs-newtab-button {
  visibility: collapse;
}

[Infocatcher]

With d6e3449 commit supported following:
[Tabs][New Tab][New Private Tab]
and
[Tabs][New Private Tab]

Unfortunately there is no easy way to support [Tabs][New Private Tab][New Tab] too.
So now we have following:

 Tabs area                                                                    Customizable toolbar area
┌────────────────────────────────────────────────────────────────────────────┬────────────────────────────┐
│First tab] ... [Last tab][New Tab][New Private Tab][<-- flexible spacer -->]│[New Tab*][New Private Tab*]│
└────────────────────────────────────────────────────────────────────────────┴────────────────────────────┘
 * – real toolbar buttons

And buttons after last tab are visible only until all tabs are visible too (and then becomes visible real buttons).

[Infocatcher]

Test version: private_tab-0.1.4pre7-fx-sm.xpi (source).