plugin: add API for plugins to provide status info messages

This can be used to store informational messages, errors or warnings which can later be shown to the user in gst-inspect-1.0, which can be useful for plugins that expose elements dynamically based on external libraries or hardware capabilities.

Status messages can then provide an indication as to why a plugin doesn't have any elements listed, for example, or that the underlying library is a build with missing features.

FIXME

• squash
• remove coreelements use of API
• use API in more plugins (maybe others want to do that @seungha.yang @vjaquez to see how/if it works for them)

Screenshot

(Exact layout/placing of this can be bikeshedded later)