CSS styling is chaotic

[haydar-metin]

The CSS is loaded multiple times + not in the correct order.

• [x] SVG filter var(--svg-def-filter-drop-shadow) makes problems for elements (they get blurry in some cases: Connect element in package to outer element)
• [ ] Icons can be removed (#149)
  • client/packages/uml-glsp/css/colors/sequence/diagram.css
  • client/packages/uml-glsp/css/colors/sequence/icons-dark.css
  • client/packages/uml-glsp/css/colors/sequence/icons-light.css