v1.90.7

Change-log:

• Update readme to shallow clone imgui submodule.
• Fix installation of pdb files to make it optional.
• Bump imgui submodule from v1.88.0 to v1.90.7.

Thanks @stevewgr for #7

v1.88

Change-log:

• 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.

v1.67

Fix DirectX 12 shared library build.

Explicitly linking library to avoid linking errors.