UI to export / convert sprite to multiple files or create texture atlases

Never wrote about this, but I always wishes for a possible UI to export sprites to multiple outputs or create texture atlases (#292) from multiple sprites.

There are two possibles ideas:

• Create an UI to connect input and output nodes to specify all the conversions between source sprites, and output // #2966
• Create an easy way to specify the filename formats in File > Export (related to #1252) + macro recording (#475), in this way the export procedure would be just a recorded macro (Lua script)