Add the Application Layer with Window Management

[mallory-scotton]

Closes #18

Type of Change

• 🐞 Bug Fix (non-breaking change which fixes an issue)
• ✨ New Feature (non-breaking change which adds functionality)
• 💥 Breaking Change (fix or feature that would cause existing functionality to not work as expected)
• 🧹 Refactor (code cleanup, architectural improvement, or progressive abstraction)
• 📖 Documentation Update
• 🧪 Tests / CI / Build System (CMake) Update

Description

Checklist

• My code follows the gp-engine C++ Coding Standard.
• I have compiled the engine successfully using the modern C++23 toolchain (GCC 13+, Clang 16+, or MSVC 19.38+) and CMake.
• I have performed a self-review of my code and added comments, especially in complex areas like shader programming or math-heavy algorithms.
• I have updated the relevant API reference or learning path documentation (if applicable).
• My changes generate no new compiler warnings in the build log.
• I have read and agree to the Code of Conduct and Contributing Guidelines.

GP Contributor License Agreement

I give to the Graphical Playground Team, LLC permission to license my contributions on any terms
they like. I am giving them this license in order to make it possible for them to accept my
contributions into their project.

As far as the law allows, my contributions come as is, without any warranty or condition, and I
will not be liable to anyone for any damages related to this software or this license, under any
kind of legal claim.