gst-device-monitor: Do not serialize string properties in the output
These properties are supposed to be readable by humans, and extraneous
quoting makes it harder to figure out what the raw string is.See: #922
gst-device-monitor: Detect the shell and quote launch string
GstValue serialization is not the same as shell quoting, and raw
strings will also not yield a usable launch string. We need to detect
the string and quote the launch string.Fixes #922