As the title said.
Requested in plasma-mobile/plasma-settings#17
I marked it as a draft because there is a small regression which removes vertical alignment of right icons.
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.
