Disable thumbnails capturing from private tabs

[Infocatcher]

(from #56 (comment))

See chrome://browser/content/browser.js

let gBrowserThumbnails = {
  ...
  _shouldCapture: function Thumbnails_shouldCapture(aBrowser) {
    // Capture only if it's the currently selected tab.
    if (aBrowser != gBrowser.selectedBrowser)
      return false;

    // Don't capture in per-window private browsing mode.
    if (PrivateBrowsingUtils.isWindowPrivate(window))
      return false;