UI: Make selecting JAH decks a single step action rather than two steps

[NateTheSage]

So, since JAH has 31,700 cards spread across 205 decks at the time of this writing, it's a little onerous to have to scroll through the list several times to pick out the decks you want and hit that plus to get them into your game.

It's lots of clicking that could be halved by making the dropdown a multiselect rather than a select-and-add configuration.

Example:

(Forgive my hackney'd example!)

Since the dropdown is left-aligned, the checkboxes would be most visible to the left of the deck name. It certainly wouldn't be a hard requirement though, since the deck names already show a check when they've been loaded in.

Just my two cents! Other than this one small request, this has been a wonderful thing to have for friends coming over to visit!