Move styling of rounded tag description boxes to Tag classes.

Introduce .rounded-box CSS class for the generic rounded style.
Then add additional classes to the actual divs to apply custom
styles through @css attribute.
Rename _signatures.scss to _tags.scss.

This file now contains more than just signature styles.
Move text base color style to .x-panel-body.

This way the color of individual p can be more easily overridden.
Move private class styles to @private Tag class.
Fix styling of singleton and enum class headers.
Move enum box styles to @enum Tag class.