Improve SVG export

During SVG export a new CSS is always created.
In practice, however, an existing CSS must be selected.
At the moment, each SVG would have to be edited and the central CSS applied.

The CSS should also be able to be exported separately.

SVG and CSS do not use the style names of the DESIGNER.
This makes it very hard to work with the CSS.

For objects without DES styles a CSS style will be created nevertheless.
These objects should only get attributes.
Since CSS styles are randomly named, such CSS styles cannot be addressed by a central CSS.