Document Sprite.filename specifics

[Arecher]

• Added examples and explanation for the three different strings that sprite.filename can return (filepath, name only and "")
• Added link to app.fs.fileTitle to show how to get the name of the Sprite
  Since filename sounds like it would return this, rather than the filepath
• Added note that switching filenames for opened sprite only changes where the file is saved on subsequent save actions, and does not switch to the file at the new path.
  I wasn't fully sure what to expect from setting a filename during runtime, and since it originally mostly mentioned it was where the file was loaded from, I wondered whether it would switch the data in the sprite if you adjusted the filename to a preexisting sprite

[dacap]

Sorry @Arecher, I've just pushed some old commits to the Sprite.filenames just today, so this PR is giving some conflicts, but probably you can just replace the latest content with your reword.

[Arecher]

Done! I'll make some changes though, because like your change mentioned, even sprites that don't have any files associated with them return their name (which seems to just be Sprite).

[Arecher]

I couldn't squash/amend the commits because one of them is a merge. Wasn't aware of that, so it's a bit of a mess of small commits now.

But the end result should be a properly merged file. I also removed my sentence that stated that Sprite(width, height).filename would return "", since that's incorrect, and added a reference to the newly added hasAssociatedFile().