Adds support for placeholder fields with dynamic slot generation.
Adds tests and fixes leak in field.
Merge remote-tracking branch 'upstream/develop' into dynamic-placeholder-field
Adds documentation.
Removes magical slotname generation in favor of explicit callables.
Removes unused import.
Moves validation for placeholder field to  init, also now explicitely checks for type string.
Merge branch 'refs/heads/develop' into dynamic-placeholder-field
Merge branch 'refs/heads/develop' into dynamic-placeholder-field