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

.github/workflows/rolling-release.yml

@@ -146,7 +146,7 @@ jobs:
       shell: bash
       run: |
         sudo apt-get update
-        sudo apt-get -y install cmake ninja-build ccache libsdl2-dev libgtk2.0-dev qtbase5-dev qtbase5-dev-tools qtbase5-private-dev qt5-default qttools5-dev
+        sudo apt-get -y install cmake ninja-build ccache libsdl2-dev libgtk-3-dev qtbase5-dev qtbase5-dev-tools qtbase5-private-dev qt5-default qttools5-dev
 
     - name: Compile build
       shell: bash

README.md

@@ -172,7 +172,8 @@ Requirements:
 Requirements (Debian/Ubuntu package names):
  - CMake (`cmake`)
  - SDL2 (`libsdl2-dev`)
- - GTK2.0 for file selector (`libgtk2.0-dev`)
+ - GTK3.0 for file selector (`libgtk-3-dev`)
+ - pkgconfig (`pkg-config`)
  - Qt 5 (`qtbase5-dev`, `qtbase5-private-dev`, `qtbase5-dev-tools`, `qttools5-dev`)
  - Optional for faster building: Ninja (`ninja-build`)
 

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()