Add ability to be included as a submodule in CMake projects

[klaussilveira]

The engine should be able to be included as a submodule in a CMake project. This allows the game developer to use his own build system, or VCS, to manage engine versions. It also gives him the ability to patch any bugs in the engine without major workflow disruptions.

Relevant discussion: https://discourse.urho3d.io/t/embedding-the-engine-as-a-submodule-and-cmake-module/1636
How rbfx does it: https://github.com/rbfx/sample-project/blob/master/CMakeLists.txt#L40