calendar: Fix initial do-not-disturb state
The do-not-disturb button and the contained switch are tied together via a bidirectional property binding.
However it still matters which objects are used as source and target, as that will determine the initial state: Right now the (unchecked) button is used as source, which means that do-not-disturb is turned off on startup.
We want the state to be preserved, so swap source and target to let the switch (that is bound to the underlying GSetting) control the initial state.
Fixes #2804 (closed)