Use lookup string callback function for `sequences_all` RNA property
`rna_SequenceEditor_sequences_all_lookup_string` using a GHash for faster lookups.
When names are changed or strips are added/removed the lookup is tagged invalid.
The next time the lookup is used it will rebuild it.
Reviewed By: sergey, jbakker
Differential Revision: https://developer.blender.org/D11544