fix datetime sync

[eneufeld]

As the datetime renderer has to use a date object instead of the string value in the model, there is an issue, that the shown date between multiple renderers bound to the same modelvalue is not synced.
This patch fixes this issue by adding a watch to the model value and trigger an update of the date obejct.

[coveralls]

Coverage increased (+0.03%) to 91.617% when pulling e50e1b8 on eneufeld:feat_datetime into a32ddae on eclipsesource:master.