Notify when the source content has changed.

And make refreshes incremental, so only new or removed items are
changed.
[datasource] Corrected emitting countChanged when removing rows
[datasource] Callback returns immediately on canceled refresh
[datasource] More verbose warning on non expected received opId
[registry] Added more debugging output

Debugging more verbose when looking up for sources
and no registry object available yet.
[registry] Added loadPluginById API

If we use the loadAll API we will get warnings on
already registered plugins.

With the new loadPluginById API, we first check if
the plugin is already loaded so we can avoid these
warnings.
Merge pull request #11 from Igalia/devel

Correct missing emission on removed rows, new registry API, souce cb to return immediately on canceled request and more debug verbosity