Add ability to set external padding

[justincbeck]

Is your feature request related to a problem? Please describe.
There doesn't appear to be a way to add external padding. If the popover is initiated from a widget that is against the side of the window, the popover gets it's padding from the arrow height. External padding should be independent of the arrow height and position, etc.

Describe the solution you'd like
The API should include values for external padding that take an EdgeInsets value.

Describe alternatives you've considered
The arrow will work for me for now although it doesn't match designs. I am looking into other ways to solve this.

Additional context