Add a sample UI for linux apps - lighting app (#24979)

* Initial version of a imgui UI in light app for linux.

Also fixed linux app shutdown to handle signals
by loop terminating instead of app killing.

* remove some fixme comments

* Fix unit tests ... with-ui variant was added

* Restyle

* Update text and remove some commented out code

* Restyle

---------

Co-authored-by: Andrei Litvin <andreilitvin@google.com>

.gitmodules

@@ -289,3 +289,7 @@
 	path = third_party/libwebsockets/repo
 	url = https://github.com/warmcat/libwebsockets
 	platforms = linux,darwin,tizen
+[submodule "third_party/imgui/repo"]
+	path = third_party/imgui/repo
+	url = https://github.com/ocornut/imgui
+	platforms = linux

examples/common/QRCode/BUILD.gn

@@ -21,7 +21,10 @@ config("qrcode-common_config") {
 static_library("QRCode") {
   output_name = "libqrcode-common"
 
-  sources = [ "repo/c/qrcodegen.c" ]
+  sources = [
+    "repo/c/qrcodegen.c",
+    "repo/c/qrcodegen.h",
+  ]
 
   public_configs = [ ":qrcode-common_config" ]
 

examples/lighting-app/linux/BUILD.gn

@@ -16,6 +16,7 @@ import("//build_overrides/chip.gni")
 
 import("${chip_root}/build/chip/tools.gni")
 import("${chip_root}/src/app/common_flags.gni")
+import("${chip_root}/third_party/imgui/imgui.gni")
 
 assert(chip_build_tools)
 
@@ -46,6 +47,18 @@ executable("chip-lighting-app") {
     "${chip_root}/src/lib",
   ]
 
+  if (chip_examples_enable_imgui_ui) {
+    deps += [
+      "${chip_root}/examples/common/QRCode",
+      "${chip_root}/third_party/imgui",
+    ]
+
+    sources += [
+      "ui.cpp",
+      "ui.h",
+    ]
+  }
+
   include_dirs = [ "include" ]
 
   if (chip_enable_pw_rpc) {

examples/lighting-app/linux/main.cpp

@@ -26,6 +26,10 @@
 #include <lib/support/logging/CHIPLogging.h>
 #include <platform/Linux/NetworkCommissioningDriver.h>
 
+#if defined(CHIP_IMGUI_ENABLED) && CHIP_IMGUI_ENABLED
+#include "ui.h"
+#endif
+
 using namespace chip;
 using namespace chip::app;
 using namespace chip::app::Clusters;
@@ -81,7 +85,16 @@ int main(int argc, char * argv[])
     }
 
     LightingMgr().Init();
+
+#if defined(CHIP_IMGUI_ENABLED) && CHIP_IMGUI_ENABLED
+    example::Ui::Start();
+#endif
+
     ChipLinuxAppMainLoop();
 
+#if defined(CHIP_IMGUI_ENABLED) && CHIP_IMGUI_ENABLED
+    example::Ui::Stop();
+#endif
+
     return 0;
 }