play with idea of suggested insertions which can be used by completions.

these suggestions would be removed once a completion is selected.
move cursor to beginning.
smoother transition on highlight borders.
allow regex failure so we can type things like \w+.
try to limit drawing to visible area and let gtktextiter find the tags.
be relaxed about regex compile.
button click unselects the overlay shading.
escape just hides overlay, not back to insert. second escape does that.
only update overlay if search string was modified.