Standardize method for getting database object in models

We have a ```getDbo()``` method in JModelLegacy (https://github.com/joomla/joomla-cms/blob/staging/libraries/legacy/model/legacy.php#L373) . This effectively is still a proxy to JFactory::getDatabase() - however it can be injected into the class and I see us standardising on using this method as us really promoting the use of injection into MVC classes instead of using these static JFactory methods.