Better support for services in yaml

[Haehnchen]

origin https://github.com/Haehnchen/idea-php-symfony2-plugin-doc/issues/22

With Symfony 3.3 it is quite common to simplify service definitions from this

services:
    myservice:
        class: App\Whatever\MyService

to this:

services:
    App\Whatever\MyService: ~

The problem currently is that in this case the plugin does not suggest class names. it only does that for the class: definition.

[Haehnchen]

OK
services:
    <caret>: ~
    Foobar<caret>: ~

# currently not possible without hacks
services:
    <caret>