Until now, if a plugin adds a custom data item provider that adds
a root item to the browser model, the new root data item would not
get added and a restart of QGIS was necessary.
Adds the data provider key of the data provider
that created the item.
Also keeps a map between data *item* provider name
(AKA data item provider "key") and the data provider
name (AKA data provider "key").
These providers will be used to control how the browser data items
behave within GUI, and to allow separation of GUI related
properties of browser items from the core code.
A new registry QgsDataItemGuiProviderRegistry has been created
(modeled off QgsDataItemProviderRegistry), with an application
wide instance available from QgsGui::instance()->dataItemGuiProviderRegistry()
