add upwardPreference and downwardPreference in PullDownMenuPosition.

[parcool]

When the button is in the lower half of the screen and there's sufficient space below it to display the popup menu, use [downwardPreference] to position the menu below the button. The same logic applies to [upwardPreference].

[notDmDrl]

Hey @parcool,

Thanks for the PR. I'm sorry it took a long time for me to respond.

I've noticed a few issues with the current implementation.

The menu is not shown when using upwardPreference, and the button anchor is at the bottom of the screen. I assume it renders somewhere beyond the screen.

Screenshot

The menu clips it's size when using downwardPreference, and the button anchor is at the upper half of the screen.

Screenshot

You can verify this behavior by running the example app -> examples -> PullDonwMenuPosition category.

I have not looked at the code yet, so I am not sure what the problem is with both added options.