[Idea] Transfer search by multiple specialties

I saw this at the forums (post 17073022.1) and I brought it here.

There are some specialities which fit better to a certain kind of player. E.g., when I'm looking for wingers I search for headers and quick wingers only. And when it comes to very rare players I could either scan the market daily or install two player search profiles. And this is what brought me to the following idea. Why not making it possible to choose two specialities at once?

Currently you cannot choose multiple specialties when looking for a player in the transfer page.

So I was wondering: Why not choosing which specialties show/hide?

My first approach was this [https://github.com/minj/foxtrick/pull/1502](pull request). I added multiple check buttons to the current Foxtrick's transfer filters, so you could choose which specialties to hide on the transfer page results.

As MJ told me, using a multiple select box could be better. Also, not sure if it's better (or intuitive) to use a box for "Show" filters or for "Hide" filters (I think that a "show" instead of "hide").