Blender Git Loki

Fix [#30136] Adding Sequence Plugin fails.

Problem was in how file selector is called by the effect_strip_add operator invoke. WM_operator_filesel() is not suited, as it directly calls exec if filepath is already set, and sequencer_generic_invoke_xy__internal() will set that path unless otherwise told. So using rather the same way to do as the one used by the other "strip_add" operators...