Fix #106740: VSE Image sequence can't be loaded from python

Caused by introduction of `SEQ_SINGLE_FRAME_CONTENT` flag (66eedc542b).
The flag was not updated in `SequenceElements` `append` and `pop`
functions.

source/blender/makesrna/intern/rna_sequencer_api.c

@@ -580,6 +580,8 @@ static StripElem *rna_SequenceElements_append(ID *id, Sequence *seq, const char
   BLI_strncpy(se->name, filename, sizeof(se->name));
   seq->len++;
 
+  seq->flag &= ~SEQ_SINGLE_FRAME_CONTENT;
+
   WM_main_add_notifier(NC_SCENE | ND_SEQUENCER, scene);
 
   return se;
@@ -608,6 +610,10 @@ static void rna_SequenceElements_pop(ID *id, Sequence *seq, ReportList *reports,
   new_seq = MEM_callocN(sizeof(StripElem) * (seq->len - 1), "SequenceElements_pop");
   seq->len--;
 
+  if (seq->len == 1) {
+    seq->flag |= SEQ_SINGLE_FRAME_CONTENT;
+  }
+
   se = seq->strip->stripdata;
   if (index > 0) {
     memcpy(new_seq, se, sizeof(StripElem) * index);