main: Handle terminating event

This breaks start/terminate/stop cycle when moving windows to another
output. Due to the way that dynamic movement of windows takes place we
also need to handle termination.

This was an oversight from when adding gRPC proxy.

Fixes: 5d6e250cf1175ede ("AglShellGrpcClient: Activate windows with gRPC")

Bugt-AGL: SPEC-5049
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: Ic19a13fce155e706fb1f9e9dac798667a592a48a