This commit makes use of the ownership,shared and ownership,unique
events from Efl.Object in order to avoid the C# wrapper from being
collected while C code holds a reference to the object.
For example, creating a list of items in a for loop and attaching events to
them would fails without this commit, as the C# GC may collect the wrapper.
The basic idea is that we use a WrapperSupervisor, which is stored in
the Eo data storage, with a GCHandle allocated for the lifetime of the
underlying Eo object. This supervisor takes care of holding either a
weak C# reference (when in unique mode, allowing the wrapper to be GC'd)
or a hard C# reference (when in shared mode, making the wrapper
non-collectable while the Eo has extra references).
One limitation is that object graphs can leak if a shared object in the
graph - an Eo child for example - stores a hard reference to another
object in the graph as a C# field. In this example, this causes the
parent to always have a hard C# reference (from the child) as the child
is non-collectable due to the parent holding an Eo reference to it.
Depends on D8678