Regroup constants

Groups operand constants in a new class to don't define different constants in form filter types classes and in expression class that aim to do the same things.

@zerkalica I saw you left a [comment](https://github.com/lexik/LexikFormFilterBundle/commit/a3e53870eea32a3a9beb5d554507352337cbfa79#L14R9) about constants so if you have any observation feel free to comment the PR :smiley: