Use gtk3 for nativefiledialog

Rational: gtk2 is not maintained anymore and the change was simple.

- Update of nativefiledialog/CMakeLists.txt in order to detect and use
  gtk3
- Updated a few files referencing gtk2, especially:
  - github workflow: I tested that the apt-get update works on ubuntu
    18.03 in a docker container
  - Readme.

This was tested in the following nixpkgs PR: https://github.com/NixOS/nixpkgs/pull/107878

dep/nativefiledialog/CMakeLists.txt

@@ -14,12 +14,13 @@ if(WIN32)
 elseif(APPLE)
   target_sources(nativefiledialog PRIVATE src/nfd_cocoa.m)
 else()
-  find_package(GTK2 2.6 COMPONENTS gtk)
-  if(GTK2_FOUND)
+  FIND_PACKAGE(PkgConfig REQUIRED)
+  PKG_CHECK_MODULES(GTK3 REQUIRED gtk+-3.0)
+  if(GTK3_FOUND)
     message("Using GTK for nativefiledialog")
     target_sources(nativefiledialog PRIVATE src/nfd_gtk.c)
-    target_include_directories(nativefiledialog PRIVATE ${GTK2_INCLUDE_DIRS})
-    target_link_libraries(nativefiledialog PRIVATE ${GTK2_LIBRARIES})
+    target_include_directories(nativefiledialog PRIVATE ${GTK3_INCLUDE_DIRS})
+    target_link_libraries(nativefiledialog PRIVATE ${GTK3_LIBRARIES})
   else()
     message(WARNING "Unknown platform for nativefiledialog")
   endif()