Add an option to the EV editor

[MagicalChocolate]

I think the EV editor should show how many EVs left we can still use, similar to Pokemon showdown's EV editor. PkHex's EV editor shows in total how many we have but doesn't show how many EVs we can still use 

[Kaphotics]

Added in latest commit, just hover over the EV Total box to see the total unassigned EVs. It's just 510-total.