Make sticky scroll take into account line heights when the cursor moves

[aiday-mar]

Currently when the cursor moves, we always reserve maxNumberOfStickyLines*heightOfStickyLine above it, so the cursor does not end up under the sticky scroll widget. Since now lines can have variable line heights, the previous heuristic does not work. We should make the sticky widget be aware of its own height at a specific cursor position, so that the cursor never appears behind the sticky widget.