CMake: Add FindGTK3.cmake file

cmake/modules/FindGTK3.cmake

@@ -0,0 +1,12 @@
+find_package(PkgConfig QUIET)
+
+set(__gtk3_required_version "${${CMAKE_FIND_PACKAGE_NAME}_FIND_VERSION}")
+if(__gtk3_required_version)
+    set(__gtk3_required_version " >= ${__gtk3_required_version}")
+endif()
+pkg_check_modules(GTK3 QUIET "gtk+-3.0${__gtk3_required_version}" IMPORTED_TARGET)
+
+if (NOT TARGET PkgConfig::GTK3)
+    set(GTK3_FOUND 0)
+endif()
+unset(__gtk3_required_version)