[Feature] Add a "Load position" button to board editor menu

[tom-anders]

This button should open a selection popup with positions to choose from.

Positions should include openings, endgames, and a "clear board" option.

For reference, here are the endgame and opening positions that the old app supports: https://github.com/lichess-org/lichobile/tree/master/www/data - We could either load this file as json, or convert it to Dart code.

@veloce I think this is a candidate for the "Good first issue" label

[ijm8710]

Probably makes sense to also ticket that eventually have a variant filter as well (just for parity with web)