Restrict collapsed items in Navbar to only those in .nav-collapse, and buttons in Navbar.

Created by: tierra

Also fix styles for buttons in the Navbar for proper spacing and alignment along with their Navbar-specific dropdown styles.

This is completed work based on issue #2895. There are two distinct parts to this pull request.

First, nav items in the Navbar are now specifically scoped so that only items contained in .nav-collapse are actually collapsed in < 980px screens rather than all nav items (including all dropdown types). This change was required to implement the second part of this feature.

The second part adds appropriate Navbar-specific styles to buttons (including dropdown buttons) so that they can be used in the Navbar. There is one caveat to this feature though.

• Unfortunately anchor buttons could not be supported (only input.btn, button.btn and .btn-group is supported). The Navbar contains numerous customized anchor styles that override the base button styles effectively killing anchor buttons in the Navbar. Hopefully someone else can figure out a clean fix for this in the future.

I have modified the fluid.html example to use a button dropdown in place of the navbar-text to illustrate a useful account dropdown that remains shown in responsive designs even on mobile screens.

For whoever is reviewing these changes, I should also explain that the extra li.dropdown scope added to dropdown styles were only to styles meant specifically for .nav dropdowns, but not button dropdowns (mostly dealing with the appearance of the caret). The added scope makes it possible to add button dropdowns either outside or inside of a .nav item without any issues or differences, which might be desired depending on the Navbar layout.