Initial step towards supporting multiple icon styles via different
directories.

Simplifies directory structure and allows for different controllers to
use different stick icons.

Necessary to support different resources for different styles.

Kenney's latest high-resolution input prompts asset doesn't require the
icons to be completed in an editor before use. Completing the pixel
icons in this commit ensures that the pixel and high-resolution icons
will provide the same out-of-box experience.

- Ensure that get_setting() uses a path that exists after refactoring.
- Set ProjectSetting entry to "" because it should now only be set for
  custom resources, and will otherwise conflict with icon styles.

Includes some placeholders for prompt icons that are included in
Kenney's pixel prompts but missing from the high-resolution version.

In order for the "Reset" button in the ProjectSettings to work
correctly, the metadata for each project setting has to be set
unconditionally. Only the value itself should be guarded by a
has_setting() check.

Sometimes, the PromptManager node seems to be removed from the tree
before the InputPrompt nodes. When this happens, the editor exits with
an error.