We so far have been recreating the background elements for all the item throughout the shell. This merge request simplifies this by unifying these elements into a single component, making is easier to keep things at a consistent design while also being able to adjust things when needed in the future.
Per request, I tried to keep thing looking mostly the same as before.
The first picture is what the action drawer looks like now, the one after is with these changes.
Adjusted the colors of the notifications and media controls to help them fit in better together.
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.
This allows us in the future to use mobileshell without having the WindowUtil singleton loaded (which does a bunch of wayland calls that aren't necessary for most applications).
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.
