Add filler lines for visual alignment in diff views

Use pixel-spacing TextTags (pixels_above/below_lines) to keep left and
right panes vertically aligned when line counts differ.  A transparent
DrawingArea overlay draws thin colored indicator lines at each filler
gap — orange for deletions, green for insertions, yellow for replacements.
Works in both 2-way diff and 3-way merge views.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: joske