Wrapper for imgui library that makes it a legit CMake component

To reduce copy-pastes when I want to use explicitly in my CMakes.

If you don't want to use the version used here (0.11.3):

• The ocornut-imgui directory contains the module to the original imgui repository.
• Type git submodule update to download it for your machine.
• Checkout the commit you want to have.
• Run setup-imgui-allegro.bat or setup-imgui-allegro.sh from the repo directory, to copy the files with proper allegro context (currentlly only allegro, because I'm using this one).

For other usages just make it a submodule in your project and include it's directory in your project.

License is the same as in the imgui library by Omar Cornut. Please refer to the LICENSE file for further details.