Various additions/fixes

[CrashTestJava]

• Document sprite.isValid
• Document websocket error param, closes Document error message param #145
• Document ui parameter for all commands, closes Document command ui parameter when 1.3.16 releases #140
• Fix app.clipboard title error
• Complete missing savefile info, closes Fix savefile command typo/issue #149
• Document Color{tile=index}, closes Document Color{tile=index} constructor #144
• Update changes.md
• Document sprite.useLayerUuids

[Gasparoken]

LGTM👌🏼!
Remove it unless you're still working on it:

• Document ui parameter for all commands, closes Document command ui parameter when 1.3.16 releases #140

[CrashTestJava]

I initially (after I had looked into the PR a little more) didn't think it was necessary to document the UI issue as the annotations ckaiser added to the command page mention the ui parameter already, and it wouldn't really matter for the commands that don't have UI; that's why I didn't delete/check it off of the checklist. In hindsight though it's probably a good idea for me to look through the commands and add the ui parameter/annotation to any command with UI that doesn't have it already. I'll add that to the PR soon.