Top Posts Widget: refactor to allow conditional loading of the css

Move the CSS file to its own folder, to stay consistent with other
widgets and keep things organized

Use wp_enqueue_scripts instead of wp_print_styles so the css is
only loaded when necessary
Top Posts Widget: update path for CSS concatenation
Top Posts Widget: use dashes instead of underscores in slug
Merge pull request #1185 from jeherve/update/top-posts-widget

Top Posts Widget: refactor to stay consistent with other widgets