Improve admin experience for Event Content Listing

[jmurty]

Improve the admin experience for the Event Content Listing content plugin:

• horizontal filter for limit_to_types choice
• verbose_name for Content Type
• default (required) value for No Items Message.

[cogat]

Also, it appears that the date controls don't update the value in the field (before saving), though if one saves the values appear to have been saved.