This adds support for touchpad scrolling in various shell components, such as the lockscreen, homescreen and action drawer.
Currently TODO because it appears to be very buggy when there is a control underneath that also accepts touchpad input (ex. flickable). The touchpad scroll start appears to get called by Qt, but not the end event, so I am unable to "let go" of the flick. Not sure if it's a wayland issue.
This also appears to not work in the nested KWin session, not sure if it's because of libinput or something
Fixes an issue where if the media controls were active and the action drawer was flicked opened, sometimes it would not fully open.
Changes include:
1. Media controls will now no longer animate its height if `implicitHeight` is 0
2. The action drawer animation code was reworked to allow its height to change dynamically.
Move from a C++ library + QML plugin to a QML plugin only for simplicity, since the homescreen switching architecture will be done from Plasma, and so use of the shell library only needs to be from QML.
