[BUGIFX] Clipboard panel as web component

The clipboard component still suffers a lot of
problems, due to its static integration in the list
modules.

For example, when clearing the clipboard, it's no
longer possible to copy / cut a record via context
menu, since the the previous GET parameters are
included in the return URL, directly reverting the
operation.

The list modules should therefore not longer handle
any clipboard logic directly, especially not via GET.

This patch does the first step towards this goal and
introduces a dedicated endpoint, only available via
POST. The clipboard panel, used in recordlist and
filelist is furthermore now integrated as a web
component, making any form submit in the main module
superfluous.

Additionally some related bugs are fixed:

* Elements moved to clipboard via context menu can now
  be remvoed from clipboard again
* Non related elements are now always properly styled
* File elements now correctly link to their parent folder in filelist
* Buttons are only shown...