[l10n] Updated German translation
Completion: rename activate_current_proposal()
Completion: hide window after proposal activation

Since the interactive completion is blocked during the proposal
activation, the window can not reappear. So it's simpler to hide the
window after the proposal activation.
Completion: simplify get_selected_proposal()

Remove the 'iter' parameter, used nowhere.
Completion: connect signals with G_CONNECT_SWAPPED

It permits to call directly the right function.
Completion: remove info_toggled_cb()

Use g_object_bind_property() instead.