This fixes#36574 by showing an error in the data
items when the layer cannot be opened.
In the same way, similar errors now bubble up
to the message bar from data source select dialog.
When the data source select dialog is closed and reopened it's much more
likely that the user wants to add a different layer than last time he was using
the dialog. So instead of offering him to add the same layer again, the selection
is cleared and he is requested to make a new selection.
- grass data item provider fixes
- removed QgsProviderMetadata constructor (with std::function / PyObject) due to sip errors (api break)
- reverted DataCapability move to Qgis - back to QgsDataProvider (avoiding api breaks)
- WidgetMode enum documentation
- sipify monkey patching fix
- renamed WidgetMode's "None" to "Normal" value - in python None has special meaning (api break)
