[#17288] Sequencer API: added a method, a geter/setter, the blend modes dict and corrected a malfunction on audio strips blend mode

from Luca Bonavita (mindrones) 

- adds the method "rebuildProxy()" useful to rebuild all the strips at once:   the user can do

- adds a BlendModes dictionary under the Blender.Scene.Sequence module: the  user can see the blending option with

- adds the getter/setter "blendMode"

- adds a function seq_can_blend in sequence.c as requested by Peter, useful for these purposes but also to solve a bug
after

- the bug is you can apply blend modes to an audio strip that doesn't make sense: changed the test and now you cannot
assign blend mode other than Replace to audio strips

Omitted DNA cleanup part since its only whitespace and Id prefer to have a useful "svn blame" output.

source/blender/src/sequence.c

@@ -2380,6 +2380,16 @@ ImBuf *give_ibuf_seq(int rectx, int recty, int cfra, int chanshown)
 	return i;
 }
 
+/* check used when we need to change seq->blend_mode but not to effect or audio strips */
+int seq_can_blend(Sequence *seq)
+{
+	if (ELEM4(seq->type, SEQ_IMAGE, SEQ_META, SEQ_SCENE, SEQ_MOVIE)) {
+		return 1;
+	} else {
+		return 0;
+	}
+}
+
 /* threading api */
 
 static ListBase running_threads;