Alpha display changes:

Previous alpha-overing on black for RGB display wasn't so much useful
for artists, changed in a way:

- Made RGBA display default for node editor backdrop and image editor,
  so it'll be clear that image does have alpha channel
- RGB display will ignore alpha channel completely

Reshuffled buttons for RGBA/RGB so now the order is following:
    RGBA | RGB | Alpha | Z

Still to come: startup.blend shall be altered to make RGBA default.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54185 954f8c5b-7b00-dc11-b283-0030488c597c