Specify domain name of switchboard and plugs

[tintou]

A recommended GLib way to use and debug applications & plugins is to use logging domain.
We should really start to add the logging domain to switchboard and its plugs.
Two main reasons:

• You then know where a debug message comes from.
• You can run G_MESSAGES_DEBUG=my-log-domain-name switchboard to only show the wanted debug messages.

How to do it?
Simply add this in the beginning of our files:
#define G_LOG_DOMAIN "my-log-domain-name"
public const string G_LOG_DOMAIN = "my-log-domain-name"
Or define it project-wide using meson flags.

[arshubham]

@tintou I would like to take this up. Any decision been reached over this?