Enhancement: Redesign main layout

We've started to implement the redesign by adjusting the sidebar,
topbar and appswitcher.
While doing so, we also removed the `vue2-touch-events` dependency.

https://github.com/owncloud/web/issues/6036
https://github.com/owncloud/web/pull/6086
https://github.com/owncloud/web/pull/6222
https://github.com/owncloud/web/pull/6228
https://github.com/owncloud/web/pull/6360
https://github.com/owncloud/web/pull/6365
https://github.com/owncloud/web/pull/6366
https://github.com/owncloud/web/pull/6386
