Add the is_frozen decorator

There is a check in several places about the application frozen state: "if not frozen, then do nothing, else continue".

To ease the handling of those calls and to have less code to write, we could add a decorator if_frozen.

For instance, this code:

def foo():
    if not Options.is_frozen:
        return
    # Do something

Will be turned into this more readable code:

@if_frozen
def foo():
    # Do something