Custom trigger value widgets

With the hook recently introduced we are able to use specialised trigger value widgets.
Currently there are only simple textfields as defaults and for some fields select lists available.
But if we would add some keys in the hook, we could provide the possibility to use a custom trigger value widget for each field widget.

To achieve this, we have to add the following keys:

• trigger_value_widget: A callback used to create the trigger value widget
• trigger_value_widget_multiple: A boolean indicating whether the trigger value widget is able to handle multiple values at once (e.g. select lists are able to, but textfields aren't)
• trigger_value_widget_submit: Basically creates a string out of the value of the widget created by trigger_value_widget, that can be written to the database
• trigger_value_decode: Decodes the trigger value in the database to a string or array of strings that can be used in a #states array