WIP : update icons for auto_index

[sblondon]

This PR is a work-in-progress about bug report #45525.

The updated icons come from material-design provided by Google under Apache licence (https://material.io).

You can see the modified icons by watching icons/README.html: only some .png files are modified. They can be compared the equivalent .gif files (which are not modified).

I have several questions:

• are you interested to update the icons?
• currently the size of the icons are not exactly the same (20*22 for the older icons, 24*24 for the newer ones). Is it ok or keeping the same size is mandatory?
• I don't understand what it is the goal of the black background for the icons in the README.html. Has someone some info about it?
• Materialdesign provides .svg format. I think it would be better to use .svg files instead of bitmap files. However, I guess it's not possible currently to protect backward compatibility. Is it possible for httpd 2.6 release?

Don't merge it now, it needs still work.

[sblondon]

This is a preview showing the diffenrence between current icons and material icons. It's a partial screenshot of the README.html: