[Widget] Add interval to all widgets

[chrishelgert]

we should add the interval property from DateTime to all existing widgets.
currently the users have to refresh the page manually - which could be really annoying on a tv.

possible solution:

• global (default) interval on the page
• interval for every widget (if it not exists, we could use the default interval)

[danielbayerlein]

Each widget should have its own interval. See DateTime widget for reference.