Allow existing entities to be selected through a view.

Allow existing entities to be selected through showing a view. The view should be able to be toggled and selected in the form display tab along with its arguments. Only entity reference views should be able to be selected since this is for entity reference widgets. The selector should display the view with each row being a label for a radio input. Selecting multiple entities should be allowed by selecting one at a time and clicking the 'add existing' button.