Ambig.character navigation: start point

In the first implementation of the navigation (UGENE-5628) in case of none selected character the search starts from the first visible character and continues to the left and then down.
So if user actually sees the ambiguous character and it is not in the first visible sequence - the button "Next ambiguous" will not find that visible character but the next in the first visible sequences so the visible area will be changed.

Add searching for ambiguous characters in the visible area before scanning the whole alignment.