Add CMake project

Export Dear ImGui as CMake's ImGui package.

Options:
- ImGui_USER_CONFIG;
- ImGui_EXAMPLES;
- ImGui_BACKENDS;
- ImGui_MISC;
- ImGui_3RDPARTY;
- ImGui_OPENGL_LOADER;
- ImGui_FREETYPE;
- ImGui_TOOLS;
- ImGui_PACKAGE.

Export targets:
- ImGui::Core;
- ImGui::ImplGLUT;
- ImGui::ImplSDL2;
- ImGui::ImplSDLRenderer2;
- ImGui::ImplSDL3;
- ImGui::ImplSDLRenderer3;
- ImGui::ImplGlfw;
- ImGui::ImplOpenGL2;
- ImGui::ImplOpenGL3;
- ImGui::ImplVulkan;
- ImGui::FreeType;
- ImGui::StdLib;
- ImGui::BinaryToCompressedC.

Import targets from:
- build directory;
- installed package.

Examples:
- example_null;
- example_glut_opengl2
- example_sdl2_sdlrenderer2;
- example_sdl2_opengl2;
- example_sdl2_opengl3;
- example_sdl2_vulkan;
- example_sdl3_sdlrenderer3;
- example_sdl3_opengl3;
- example_sdl3_vulkan;
- example_glfw_opengl2;
- example_glfw_opengl3;
- example_glfw_vulkan.

Presets:
- vcpkg (require $env{VCPKG_ROOT});
- emscripten (inherits vcpkg and require $env{EMSCRIPTEN_ROOT}).

examples/example_null/CMakeLists.txt

@@ -0,0 +1,21 @@
+cmake_minimum_required(VERSION 3.5)
+
+project(example_null
+    LANGUAGES CXX)
+
+find_package(ImGui CONFIG REQUIRED
+    COMPONENTS Core)
+
+add_executable(${PROJECT_NAME} main.cpp)
+
+if(EMSCRIPTEN)
+    set_target_properties(${PROJECT_NAME} PROPERTIES
+        SUFFIX .html)
+endif()
+
+target_link_libraries(${PROJECT_NAME}
+    PRIVATE ImGui::Core)
+
+include(GNUInstallDirs)
+install(TARGETS ${PROJECT_NAME}
+    RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}")