datepicker('setDate' null) should not set today active

[raphinesse]

To overcome the issue that ​today never appears as selected or hovered I reordered the rules for ui-state-active and ui-state-hover to be more important than ui-state-highlight. However this revealed that, as of now, it is not possible to have no active date in a Datepicker instance. Calling datepicker('setDate' null) sets today as active.

[Benjamin]

Confirmed, datepicker('setDate', null) only resets the <input>, but leaves a date highlighted on the datepicker. That's definitely a problem when using an always-visible datepicker in a web application, without necessarily having a related <input>. There's currently no way to remove the selection.

[mikesherov]

Duplicate of #8009.