Merge pull request #7 from ko4life-net/imgui-v1.90.7

Bump imgui to v1.90.7 and fix pdb installation step

Refactor and cleanup project.

+ Make proper usage of cmake's package_config.
+ Provide examples to consume the library.
+ Apply proper renaming in accordance to imgui's backend changes.
+ Inline backends include function in the same CMakeLists.txt.
+ Set default CMAKE_INSTALL_PREFIX and allow for it to be overriden
(closes #2).
+ Change imgui branch from master to docking.
+ Update readme.

TODO: Add support for more backends. Now with the new structure it
should be easier.

Fix DirectX 12 shared library build.

Explicitly linking library to avoid linking errors.