The current modal in download-actions should have an advanced mode that allows users to better control the size of the exported image. Users should click an accordion to expand the advanced mode and choose/update the following properties.

• which node to be centered (default to blank since the default mode does not center any node, should be an input RmgFields that has all nodes' ids as options)
• the left, right, top, bottom distance from the chosen node (disabled if no node is chosen)
• the ratio of the exported image (16:9, 3:2, 1:1...) (an input RmgFields that is mentioned as options, default to current width/height)
• the extra padding from the existing border (default to 0)
• the top left, top right, bottom left, bottom right position

It is obvious that some properties may conflict with others when updated, so update those as well if one is changed.

Use RmgFields for all inputs.