Add "select_mode" to determine if "selection_box.visible" is true or false

    If "select_mode" is true(tile not being dragged) then
    selection_box.visible is true. Previously this check was done using
    "current_tile == -1", which was unclear.

    It could be said that it is redundant to set "select_mode" and *then*
    set "selection_box.visible" to true, but it very clearly shows what
    is being done and when. In addition, checking select_mode may be used
    later on with a completely different purpose.