Complete Command documentation

[ckaiser]

Adds documentation for every command, improves wording and fixes typos. Includes annotations for commands that require the UI or might use it optionally.

TODO

• Make sure the new formatting is compatible with the Aseprite website.
• Make the layout of all the command arguments consistent.
• Make how we show argument types consistent.
• Add a short description of the commands that have separate documentation pages in their title .
• General grammar/spelling pass

[dacap]

I think there is no need to talk about the "input chain" concept (which is something internal). The Esc key / Cancel command will deselect anything we have selected (app.range, app.sprite.selection).