Update and clean-up files, use relative search path for XML files, open URL in browser

* Update L10n, Doxyfile and XMLs.
* Fixes in debian/*.
* Clean-up unused parameters in frmabout.*, userlexer.cpp (eliminates remaining warnings during build).
* Use relative search path for XML files (fixes <a href=https://bugs.launchpad.net/notepadqq/+bug/1326241>#1326241</a>).
* URL in frmAbout dialog now opens in webbrowser.