Theme: Improve default theme styling

Theme: Improve default theme styling

Changes tooltip to use the ui-widget-shadow class, which now applies the
box-shadow style. .ui-widget-shadow was created when box-shadow wasn't
available. By now, there's no point in faking a custom shadow anymore.
This removes the only non-structural CSS from a widget-specific file.

Updates demos to use the same font-family, removes unused images.

Will be available as the new default theme on ThemeRoller called "Base",
while "UI Smoothness" and "UI Lightness" will still be available in the
gallery.

Fixes #10617
Fixes #10880
Closes gh-1436
Ref jquery/download.jqueryui.com#248