GravityView - Gravity Forms Calendar 2.8.3

* Added icon for [GravityView 2.10](https://gravityview.co/210-admin-refresh/) widget picker
* Fixed: Calendar would improperly display entries edited using GravityView's Inline Edit extension; thank you, Thomas!

__Developer Updates:__

* Added: `gravityview/calendar/settings/search_criteria` filter to specify search criteria for form entries with events.
* Added: `gravityview/calendar/settings/sort_order` filter to specify sort order for form entries with events.

* Fixed: Calendar preview fails to load until a new Feed is saved

* Fixed: Calendar preview throws an error when GravityView plugin is not activated
* Fixed: Multiple calendars sharing a single Feed ID can't be displayed on the same page
* Improved: DB query excludes entries with empty start/end dates
* Modified: End Date is no longer and unless specified will be based on the Start Date