feat(imgui): embeddable plot widget and demo

CMakePresets.json

@@ -14,6 +14,7 @@
       "cacheVariables": {
         "CMAKE_BUILD_TYPE": "Debug",
         "PLOT_BUILD_GLFW": true,
+        "PLOT_BUILD_IMGUI": true,
         "PLOT_BUILD_EXAMPLES": true
       }
     },
@@ -25,6 +26,7 @@
       "cacheVariables": {
         "CMAKE_BUILD_TYPE": "Release",
         "PLOT_BUILD_GLFW": true,
+        "PLOT_BUILD_IMGUI": true,
         "PLOT_BUILD_EXAMPLES": true
       }
     }