Supports RGB and RGBA in ImageView

[vallsv]

This PR release constraints on setImage to allow to set RGB and RGBA images.

Few other places were fixed, like statusbar display, but not much.

Changelog:

• Added support of RGB and RGBA images to ImageView

[vallsv]

I think it is fine to back port it to 0.15, but it's up to you.

[t20100]

I made a few suggestions to support RGBA as float in [0,1] and catch malformed RGB images early.
Else LGTM, I don't see what this PR can break in current API except for the valueChanged signal when you load a RGB image, but that's a new feature so it's fine.