Optimize save time

We should make the *File > Save* operation faster, mainly for the purpose of this extension, i.e. saving huge animations created with attachments (big tiles and tilemaps of 1x1).

To do so we can think that a regular operation using the Attachment System will be as follows:

1. Most of the time saving changes in anchors/reference points or adding frames re-using attachments
2. Adding new attachments
3. Big attachments with borders that can be shrunk (a lot of space/pixel data to be saved)
4. Few attachment modifications after they are created (probably an attachment, once it's created, will not be modified in its entire lifetime)

Some tasks/ideas to improve the saving performance depending on these use cases (from simplest to the most complex):

- [x] First of all profile the save operation to check what is the main bottleneck saving this kind of files, then decide how to continue
- [ ] Using indexed color mode? // User-side workflow change, we're not sure if this is possible
- [ ] Cache the loaded tile from the .aseprite file and save the exact same bytes if the tile is not modified (no need to re-compress the data if it wasn't modified) // more memory usage when editing the sprite
- [x] Cache the whole tileset? E.g. Caching the whole tileset chunk to re-save it as-is (if we cache all tiles I think this will not add too much)
- [ ] Save a shrunken version of each tile/attachment (removing all unused borders)
- [ ] Add support for uncompressed tilemaps? As we have 1x1 tilemaps // as a prerequisite we need to remove all empty tiles (shrink the tilemap) in the extension first
- [ ] Compressing images/tiles in parallel (multiple threads compressing images in the save process)
- [ ] External tilesets: From aseprite/aseprite#977 there are plans to support external tilesets, but it's a big refactor/it might take several weeks of work/testing and we're not 100% sure if this alone will improve the performance to save files (e.g. the only way to optimize the save time using this is caching the tileset / not saving the external tileset when it's not modified)