homescreen/taskswitcher/lockscreen: trim and optimise flick containers

Hi guys, this MR trims flick containers in task switcher and lock screen and removes no longer needed things, like connections with HomeScreenState and cancelMovement() hack.
Doing so, as far as I can see, doesn't break anything and seems to give a little gain in terms of performance.
Using a screen-sized flickable should make it simpler to implement logics based on flicks and, at the same time, should solve troubles with too high scrolling velocity and multiple flicks.

Home screen container will be trimmed as well, but it needs some more work.