Harmonize how modifiers are passed

[sloria]

• Rename container in to inner. Can be done quickly after Fix propagation of options to Nested containers #1229 is merged. Done in Rename "container" attributes #1243
• Harmonize the way the modifiers can be passed (as Nested parameters, as Meta params, at load time, in schema init, as string with dot delimiter, etc.).
• Merge Schema._normalize_nested_options and Nested._nested_normalized_option into a single method.

Originally posted by @lafrech in #1066 (comment)

See also:

• Fix propagation of options to Nested containers #1066 (comment)