adding qt-editor
track-area: lets not repeat ourselves
track-area: separate methods for both scrollbars
qt-editor: only expose the rectangle of a track
editor: rename awkwardly named method

While we're at it, rename SyncDocument::getTrackOrderCount to
getTrackCount.
qt-editor: make margin of the correct size, and more
editor: remove commented-out code
qt-viewer: implement row-number coloring
qt-viewer: fixes