grpc-proxy/shell: Add sticky parameter when setting up split window

This allow to further customize how to handle the split window.

There's no implementation in the compositor for allow this functionality
to take place, but doing so can be at a later point in time.

Tagging the split window as sticky should allow activation of other
windows while keeping the split window always displayed.

Bug-AGL: SPEC-4839
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Iae4d18d2a857497c11d0e77bf43f654e2b8ed501