[ticket/12608] Improve notifications drop-down menu styling in header

PHPBB3-12608
[ticket/12608] Updated tests

PHPBB3-12608
[ticket/12608] Add styling (CSS class) options to notifications

This will allow different notifications to have different styles

PHPBB3-12608
[ticket/12608] Clean up whitespace

PHPBB3-12608
[ticket/12608] Improved LTR and various bugfixes

PHPBB3-12608
[ticket/12608] Remove somewhat redundant reported color style

PHPBB3-12608
Merge pull request #7 from PayBas/ticket/12608

[ticket/12608] Improve notifications drop-down menu styling in header