[ticket/12662] Reorganize and modernize the header navbar

https://tracker.phpbb.com/browse/PHPBB3-12662
http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=45679&start=60

__TODO:__
- [ ] Fix RTL notifications dropdown positioning
- [x] Fix IE8 notifications body overflow in compact mode
- [x] Fix username dropdown arrow when user doesn't have `<span>` around his name (when user has no color)
- [ ] Fix username dropdown arrow position and dropdown container position when `navbar-header-username-append` event is used

![updated](https://cloud.githubusercontent.com/assets/3015760/3210923/13dcb72a-eef0-11e3-85a3-b553fad8fbda.png)

Logged out
![loggedout](https://cloud.githubusercontent.com/assets/3015760/3208288/085885d2-ee28-11e3-89f8-f733f0dbb24e.png)

PHPBB3-12662
That's what i thought, before the IRC conversation. 

And for me it will be okay and probably the best way, if we don't use old names in the 3.1.0 stable, we will probably have enough old awkward names in the later versions. ^^