shell: Port to AdwToast{,Overlay}
The old in-app notification style is deprecated, and using it with our widget heirarchy required hacks that are no longer supported by GTK.
This commit replaces the old in-app notifications with toasts.
I've marked the MR as a draft, as we need to figure out a few things:
- Multiple toasts can be in the queue at once. We need to rework how we handle showing additional details to tie them to a specific toast.
- Related, this commit as-is doesn't work if we have multiple of the same type of alert.
- We need to change all of the strings - toasts are small, so our strings need to be small as well.
- We have areas where we put arbitrary text into the notifications. We need a better way to show this type of alert.