Emit `suspended` surface state
This allows applications to suspend drawing when the window is fully obscured. Currently limited to "phone" aka auto-maximize
mode.
As ever so often in phoc the actual changes are quiet minimal after cleaning up around the edges.
See individual commit messages for details.
@arun-mani-j Please have a look.
A test client is at guidog/livi!61 (merged) . With G_MESSAGES_DEBUG=livi-window
it prints a message whenever the video decoding stops/resumes.
Closes: #389 (closed)