css: icons: refactor and simplify icon imports/declarations

- better usage of icons in .nav
- fixed with inside .navbar
- better alignment in .dropdown-menu
- add .icon-browse, .icon-docs, .icon-api
- make icons in navbar fixed-width
html: fix nesting of #application
css: support button in .dropdown-menu

- make them behave like a
- also support .btn-warning
#tmp