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()
With implementation in app. This allows GUI library classes to
re-use standard dialogs which are created in app. The initial
use-case is to allow the GUI library symbol list widget to
focus/open an existing Style Manager dialog (created in app),
instead of opening a new modal style manager dialog.
Side benefit - moves some code out of the monolithic qgisapp.cpp
file.
