Fix the template not starting on Windows

Apply the changes from slint-ui/slint#2034 also
here.

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.19)
+cmake_minimum_required(VERSION 3.21)
 project(my_application LANGUAGES CXX)
 
 find_package(Slint QUIET)
@@ -17,3 +17,7 @@ endif (NOT Slint_FOUND)
 add_executable(my_application src/main.cpp)
 target_link_libraries(my_application PRIVATE Slint::Slint)
 slint_target_sources(my_application ui/appwindow.slint)
+# On Windows, copy the Slint DLL next to the application binary so that it's found.
+if (WIN32)
+    add_custom_command(TARGET my_application POST_BUILD COMMAND ${CMAKE_COMMAND} -E copy $<TARGET_RUNTIME_DLLS:my_application> $<TARGET_FILE_DIR:my_application> COMMAND_EXPAND_LISTS)
+endif()