Make listscreens example work even when backend=none

We've refactored the System hierarchy so we can make an instance of
SystemWin/OSX/X11 classes even when LAF_BACKEND=none. We've made
System::makeWin/OSX/X11() functions available as they might be enough
in certain cases where the whole Skia is not needed.

Some important changes in this patch:
* os::instance() is deprecated, replaced with os::System::instance()
* os::make_instance() is deprecated, replaced with os::System::make()
* os::System::instance() returns an os::SystemRef

Author: dacap

examples/CMakeLists.txt

@@ -15,15 +15,15 @@ function(laf_add_example name console="")
 endfunction()
 
 laf_add_example(base64 CONSOLE)
+laf_add_example(listfonts CONSOLE)
+laf_add_example(listscreens CONSOLE)
 laf_add_example(show_platform CONSOLE)
 if(LAF_BACKEND STREQUAL "skia")
   laf_add_example(allevents GUI)
   laf_add_example(complextextlayout GUI)
   laf_add_example(custom_window GUI)
   laf_add_example(floating_window GUI)
   laf_add_example(helloworld GUI)
-  laf_add_example(listfonts CONSOLE)
-  laf_add_example(listscreens CONSOLE)
   laf_add_example(multiple_windows GUI)
   laf_add_example(panviewport GUI)
   laf_add_example(shader GUI)

examples/allevents.cpp

@@ -252,7 +252,7 @@ class LogWindow {
 
 int app_main(int argc, char* argv[])
 {
-  auto system = make_system();
+  SystemRef system = System::make();
   system->setAppMode(AppMode::GUI);
 
   LogWindow window(system);