feat(overlays): Added the 'priority' parameter for overlays

[raspberry-jenshen]

Description

To sort overlays we can introduce a priority. Let's say my app contains several menus that can be displayed simultaneously, but I want my main menu displayed at first in the stack (at the bottom).

Checklist

• [x] I have followed the Contributor Guide when preparing my PR.
• [x] I have updated/added tests for ALL new/updated/fixed functionality.
• [x] I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• [x] I have updated/added relevant examples in examples or docs.

Breaking Change?

• [ ] Yes, this PR is a breaking change.
• [x] No, this PR is not a breaking change.

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