Add support for loading plugin with name of style `Vendorname/Pluginname`
Add test for setting view theme to vendor namespaced plugin.
Update App::className() to account for `Vendor/Plugin.Foo` style class name.

Added tests for other App class methods for the same.
Merge pull request #3625 from ADmad/3.0-plugin

Add support for loading plugin with name of style `Vendorname/Pluginname...