Navigation by ambiguous characters in MCA

Add buttons to jump to next/previous ambiguous characters. An ambiguous character is one of the following:

• character of extended DNA, but not standard DNA alphabet,
• character N.

The "next" search is done from left to right, from top to bottom (circularly). The "forward" search is done from right to left, from bottom to top (circularly).

If a character is selected in the Reads Area, it is used as an "anchor" from which the search is started, e.g. the "next" ambiguous character is searched in the same Sanger read to the right from the selected character, then in the read below from its start, etc.

Otherwise, if there is no selected character, the "next" search is initiated from the left upper character in the currently visible area. The "previous" search is initiated from the right bottom character of the visible area. If there are no visible characters, the search is done to the right/ to the left from it correspondingly.

In case there are no ambiguous characters, show the following info notification:

The alignment does not contain ambiguous characters.