#155 Preserve position when setting or clearing manual alignment points

[Sir Zooro]

When manual alignment points are added, kdiff always scrolls view to 1st manual alignment point set. When all manual alignment points are removed, kdiff scrolls view to beginning. This is not very user-friendly. Every time when I compare long files where many alignment points are needed, I have to remember current position when setting/clearing alignment points, and restore it later manually. Please this behavior and preserve current position in file when setting/clearing alignment points.