Change head

The head of a token can be changed by selecting it with a left mouse click and then selecting its new head. This also works for multiple selected tokens using control + left mouse click. Attach a token to the root is not yet possible because the root in the tree is no token. If the root is a token, we must ensure that it cannot be change its head to something (see issue #98).
To enable the multiple selection using the control key a key-capture directive as well as an accompanying service has been introduced and can also be used for other tasks.