Remove unnecessary properties and their getters/setters

This kind of data is mainly prepared in the controller's action now or
directly fetched in a concrete form implementation.

refs #5525
Remove getConfig and add get/setBackendConfig instead

As for every configuration form, backend forms need special population
mechanisms as well.

refs #5525
Add addSubmitButton to BaseBackendForm

refs #5525
Rewrite addForceCreationCheckbox and move it downwards

We should not instantiate form elements directly without any specific
reason.

refs #5525
Make isValid more compact and fix coding style

refs #5525