Fix: VSE: Propagate split to connected strips by default

Currently, attempting to split connected strips that are not both
selected means that only one of the strips are affected.  This violates
the "connected" principle, so this PR makes the split operator propagate
the split to connected strips by default. Holding alt returns old
behavior (ignores connections), similar to selection logic.

Pull Request: https://projects.blender.org/blender/blender/pulls/146380

scripts/presets/keyconfig/keymap_data/blender_default.py

@@ -8628,6 +8628,14 @@ def km_sequencer_tool_blade(_params):
                  ("use_cursor_position", True),
                  ("ignore_selection", True),
              ]}),
+            ("sequencer.split", {"type": 'LEFTMOUSE', "value": 'PRESS', "alt": True},
+             {"properties": [
+                 ("type", 'SOFT'),
+                 ("side", 'NO_CHANGE'),
+                 ("use_cursor_position", True),
+                 ("ignore_selection", True),
+                 ("ignore_connections", True),
+             ]}),
         ]},
     )