Make the seek-when-click off by default

Currently it's possible to seek by right-click, and select clips by clicking. There is an option to seek also when clicking, because not everybody knows that right-click seeks. We should disable this by default, and make a quick tutorial about the UI which tells about right-clicking.

The quick tutorial will have initially a single item, but we can expand it later. As a model, see the Glade quick tutorial.