Add basic Seamonkey support to titlebar customization. Fixes Issue #31.

Fix for Issue #31.

The pull request consist of:
 * 'Customize Titlebar' menuitem
 * some titlebar customizing logic
  * mostly based on `browser.js`
  * and a little bit `messenger.js` (`nightlyApp.getWindowTitleForNavigator`)
 * workaround for [SeaMonkey Debug and QA UI][1]


Please test it on Mac OS X too! It would be nice to avoid bugs like [Bug 717192][2]!




[1]: https://wiki.mozilla.org/SeaMonkey:Debug_And_QA_UI
[2]: https://bugzilla.mozilla.org/show_bug.cgi?id=717192 "Refreshed Thunderbird titlebar customization code doesn't work on Mac OS X"