VSE: Fix multicam splitting all selected strips

`split_multicam` used split operator, where if more strips than
multicam were selected, all would be split, which is undesirable.

Add `Sequence.split()` RNA API function. to split individual strips.
Function accepts `frame` and `split_method arguments`. Returns right
strip after splitting.

In case when strip being split have effects, these will be split too, so
no invalid state should be created.

Selection is not handled, this is by design up to user.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D11926

release/scripts/startup/bl_operators/sequencer.py

@@ -108,14 +108,13 @@ class SequencerSplitMulticam(Operator):
         if s.multicam_source == camera or camera >= s.channel:
             return {'FINISHED'}
 
-        if not s.select:
-            s.select = True
-
         cfra = context.scene.frame_current
-        bpy.ops.sequencer.split(frame=cfra, type='SOFT', side='RIGHT')
-        for s in context.scene.sequence_editor.sequences_all:
-            if s.select and s.type == 'MULTICAM' and s.frame_final_start <= cfra and cfra < s.frame_final_end:
-                context.scene.sequence_editor.active_strip = s
+        right_strip = s.split(frame=cfra, split_method='SOFT')
+
+        if right_strip:
+            s.select = False
+            right_strip.select = True
+            context.scene.sequence_editor.active_strip = right_strip
 
         context.scene.sequence_editor.active_strip.multicam_source = camera
         return {'FINISHED'}