This can be used to restore initial layer status or to
apply styles and other layer properties if the data
source is changed.
The idea is that the user can fix bad layers at any time,
and by setting a new datasource she probably wants
to keep the original layer properties.
The issue was that during drag&drop, we first connect to layer's signal
for the second time, but then the disconnect() call on removal of the original
layer node would disconnect all connections (not just one). For that reason
now we only connect/disconnect when the layer is in the tree only once.
