Define character position once

Currently each frame in character.json contains x and y for how to place the character.

The game should probably simply place the character's bottom right corner at the right edge of the screen on top of inventory without asking questions.

Alternately or optionally there could be a single x and y setting for the character in character.json, with option to use x and y of each frame if those are set. (Needs examples / tests though.)

Relates to #127 and perhaps #99

Start by reviewing how character.json frame positions are read and used to place characters, then compare the proposed fixed bottom-right placement with the optional single x and y setting. Resolve which behavior is wanted, including fallback rules, and add the examples and tests requested in the issue; review related issues #127 and #99 for context.

Written by the indexing model from the issue text.