Simplify event filter markup
Make sure pickable instances for each section in the event editor are available. This should be removed again in Symphony 2.4, when the event editor along with the data source editor has been overhauled.
Simplify Pickable

This commit removes the new options `always` and `multiple` again at this really bloated the plugin. As a result of this, the select box will always be visible independent of the option count.

If an extension likes to hide the select box in this case this can be done with a simple `.hide()`.

This commit also changes how Pickable stores items internally in order to make it possible to add pickable items after the plugin has been initialised.