Merge tag '2.0.0-alpha9' into develop

Version 2.0.0-alpha9
Fix ContainerPasteButtonCallbackListener.
Remove spaces from global buttons - see #426.
Add GetPasteButtonEvent::[s/g]etContainedModels() method.

Now the contained models are available within the event.
Code style.
Implement reference handling.
Rework value rendering for "show" in views.
Update some CSS and templates.
Code style.
Remove the double double point from the globale buttons.
Add the class tl_tbox for the first fieldset.
Remove some debugging entries.
Optimize css and add missing empty space to header panel.
Improve the panel function for rendering the panels. Now it is possible to add a ignore list of panel. If a panel is in the list it will be skipped.
Add the panels for the treeview.
First preview of the DC_General tree picker widget.
Fix PropertyValueInFilterBuilder.
Fix composer-plugin constraint.
Require contao-community-alliance/event-dispatcher ~1.1 now.
Require contao-community-alliance/translator ~1.0 now.