Blender Git Loki

Cleanup: Minimize seq->tmp usage

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