Using this persistent field in `select_more_less_seq__internal` was
inapropriate in this case.
Split select more/less and linked code, because it was mixed in
`select_more_less_seq__internal()`
These operators work in completely different way.
`select_linked_internal()` doesn't use seq->tmp at all.
`seq->tmp` was used to mask strips selected by operator while iterating.
Use GSet to store temporary data instead of `seq->tmp`.
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D10326
66923031e6