Operator combines more features and it wasn't very transparent which
properties are used in which feature and how they are used.
Features linked_time, side_of_frame and linked_handle are
isolated, logic moved into own function.
deselect_all property was removed as it practically equals to
!extend in all cases.
Functional change: Dragging existing selection will not change active
strip. Previously this could happen if click happened on strip that
wasn't active.
Reviewed By: campbellbarton
Differential Revision: https://developer.blender.org/D12320
e6a1d488ab