Show accordion with expansion-panels instead of tabs.
This way elements can be grouped logically.

Components with bigger scope are put to the component root, canvas, 
props panle and new-element-panel.

This is done according to the Angular best practices and minimizes
namespace collisions when using other packages which might use the same
selectors.

Technically the selectors are not even becessary since we use an Angular
component factory to create element components but it is still useful
for the DOM element naming, for debugging purposes for example.

The 2 different radio button variants (with and without images) are now 
hidden behind one radio button entry. Functionally everything else works 
the same.
Also move CSS to it's own file.