This commit adds the API that allows to set the NO_MOVE flag for a child
window. This API was missing: the NO_MOVE flag could only be set for a
Window.
If the NO_MOVE flag is set to True, (by calling
`ChildWindow::movable(false)`), then the window will not move when a
child window is dragged on.
We can see this behavior in imgui's code:
https://github.com/ocornut/imgui/blob/f0f53016/imgui.cpp#L3354
This is probably going to be controversial...
Calling end/pop in the Drop implementation causes way too many problems,
and interacts very badly with panics.
There are closure-based simpler implementations of almost everything
except parameter stacks that are still convenient to use. However, if we
end up switching to &mut Ui in most functions (very much possible in the
future!), closures will complicate matters so push/pop and begin/end
pairs are still relevant.
