Here I upload two extensions, both will appear in the menu Extensions -> Animation (layer-based).
1. Export Layers as Separate Files
This one will export each layer not hidden (unless you tick "Include hidden") to a separate .png file. You can use sub-layers to group different animations.
Example: I created an svg file with the following layers:
front
- 1
- 2
- 3
side
- 1
- 2
- 3
This extension will generate the files:
basename_front_1.png
basename_front_2.png
basename_front_3.png
basename_side_1.png
basename_side_2.png
basename_side_3.png
2. Export Layers to Sprite Sheet
This one is the most interesting: it will generate directly an spritesheet with each layer as a frame. Also will create a file named basename.txt with frames (=layers) coordinates in pixels.
Example spritesheet:

Instalation:
The extension are written in python an should work in any platform. Just copy the files inside the .zip into the extensions directory.
Feedback and suggestions are welcome.