Sprite sheet export using API

Essentially i would like to create exporter for my own engine with different file format. For that it would be really usefull if i would be able to work with sprite sheet.

My idea is something like this:
```Lua
local ss = Sprite:createSpriteSheet(rows, columns) --< Maybe some description table with params you normally enter in export dialog instead of just row and column

--! SpriteSheet minimal api:
ss.width
ss.height
ss:SafeToFile("Myfile.png")
for i,sframe in ss.sheetFrames do
--! minimal sheet frame API:
sframe.posx
sframe.posy
sframe.width
sframe.height
sframe.spriteFrameNumber
end
```

This would be enough to be able to do custom exporter with all data necessary to recreate animation in external renderer (engine)

Review the existing scripting API and the export-dialog parameters referenced in the issue, then compare them with the proposed Sprite:createSpriteSheet and sheet-frame API. Done would mean scripts can access sheet dimensions, frame positions and sizes, frame numbers, and save the sheet image for a custom exporter.

Written by the indexing model from the issue text.