Blender Git Loki

[#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.