Preview thumnbail inside datablock menu

Currently we only have list inside datablock menus.

This proposal is about adding preview(thumbnail)mode inside all datablock menus. Not only materials, but also images, textures, etc.

In fact we already have this in Blender. We only need to apply the function to all datablock menus and add the ability for user to choose between list and preview mode.

It will work well with batch-generate previews function. Currently we can batch-generate previews and it is useful later on when appending that file to another one. Why not use this feature with datablock menus too?