Save Mipmaps using QImage

When displaying the Mipmap preview we are already decompressing and converting the texture to a QImage. Let's use that to allow saving textures to Qt image formats like PNG.
This might help people who just want a specific texture to work with and do not care whether it is a real texture export into a DDS or JPEG(s).

I've chosen to go the cleanest way without having to touch a lot of code and added a "Save As..." link to the preview. As a bonus this cleans up some QSettings copy-paste redundancy.
From the current system, I'm uncertain which way I should do that. What do you think about making QTextureBox::setTexture() return a boolean that we can check in QMipmap_Preview?