Admin panel: Select annotators layout and function

In "Select annotators" dialog, annotators should be selected via tickboxes instead of arrows. Summary window should show the selected annotators.

Two new functions are needed:

• a tickbox "Apply to all in subtree" (annotators are set in all the children of this node)
• a dropdown menu "Copy from", listing all nodes with non-zero annotators (annotators are copied from the selected node)

The window for the selection of annotators should be a popup dialog, or it should follow the scroll down in the tree. Currently it stays up, so selection of annotators for reactions is cumbersome.