HTML Gallery does not include css from modules

[pStrudthoff]

When selecting something in the HTML Gallery and viewing it in the preview tab
the css from our module is not included in the iframe and thus the html is
rendered wrong making the preview pane useless.

This is a major problem. With alot of elements in the gallery not being able to
see a preview makes selecting the right element time-consuming.

Another "anoing" change from our old version 7 of OpenCms is that a preview is
only shown after selecting the element from the gallery. In version 7 there was
a preview accessible right under the GalleryListing making "browsing" the
gallery easy.

[tHerrmann]

We consider the HTML gallery a legacy feature and won't do any further improvements.