Support style self-load for spatialite/postgis

[aperi2007]

When add a shapefile to the canvas, if there is a style (qml) with the same name of shapefile it is load automatically.

It would be very useful to hav a similar action for a table-layer.

It could be use the same paradigm used for the shapefiles:

Infact as for a shapefile the style to be self-load must be available in the same folder and with the same name of the shapefile but with qml extension, for a table/view (spatialite/postgis) layer the style , to be self-load, could be available in a table with "_qml" postfix. For example for the table "rivers" the style could be retrieve automatically if exist a table with name "rivers_qml", in the same database and same schema.

[jef]

duplicate of #1127