Various

[Jopyth]

This PR contains various improvements:

• Outputs can be marked as required, non-required outputs will be shown in white instead of the current default green
• Highlighting of active connections fixed
• Simple algorithm for creating a layout which uses the vertical space available in a better way with a topological sort
• Pipeline Editor can be configured to be in a popup window in addition to the usual internal view
• Deleting stages via context menu is possible
• Connections can be marked as feedback connections (reverse to pipeline order) and are displayed differently and ignored for layout purposes
• Enables integration and configuration of custom stages through a new signal and exposing the pipeline object to the editor

Current Hack:

• Layout process is done 10ms after the pipeline editor is first opened. I did not find another reliable way to call the layout function after all stage items received their actual sizes, which is needed for any nice layout function. This could probable be improved somehow.