youtubeShell: bugfix: minimize corrective seeking

bug introduced 5673dbe6d93a6e46968607baf34011f17027bc98 @jbenet
tests: add some missing collectionShell tests

test the `switchShell` and `showView` methods
collectionShell: add offset parameter to showView

showView and switchShell now accept an optional second parameter,
offset. passing an offset will seek the selected view to that offset
when it is shown
collectionShells: add option to restart subshell

when the option `restartSubshellOnProgression` is true, a collection
shell will seek a subshell to its start when it transitions to that
shell via the `showNext` or `showPrevious` methods
collectionShell: rename mediaView option

`playOnChangeShell` -> `playSubshellOnProgression`