LadspaControl: save to dedicated DOM element

Instead of cluttering parent's attribute list, we created a dedicated
DOM element for each LadspaControl. This gives us the possibility to
save further information later.

Closes #401.
AutomatableModel: load connection settings before automation link

Controller connections got lost in case the control had automation
patterns attached and thus the loadSettings() function returned
prematurely.
AutomatableModel: eliminate return statement

There's no need for the return statement anymore when changing the
branching logic.