Code review comment for lp:~mir-team/mir/unify-pointer-button

I've thought through my concerns that related to nto storing buttons as a flag. Primarly I was worried about exotic devices and allocating button indices for them and potentially running out of flag values. I suppose if anything though such devices would not be pointers and only available throuhg the raw input API so I'm not concerned. Updated branch to use flag...required more client ABI break :)