maplibre support for Qt6 and MSVC 2019 (#125)

* [qt] Updating CMake project files for Qt6

* [qt] Updating non-test projects for Qt6 and msvc2019

* [qt] Test fixes for Qt6 and msvc2019

* [qt] Link whole test archive for Qt and msvc2019

* [qt] Fixing Qt6 headless backend renderer

* [qt] Fixes for test failures

* [qt] Expression and style parser test fixes

* [qt] Backwards compatibility with Qt5 and Qt6

Minimum version is Qt 5.4 due to use of QOpenGLWidget

* [qt] LLVM is no longer required for a Windows build

* [msvc] msvc2019 fixes for maplibre

* Update test/storage/offline_database.test.cpp

Co-authored-by: Tadej Novak <tadej@tano.si>

* Vector + const is not standard C++

Co-authored-by: Tadej Novak <tadej@tano.si>

test/util/thread.test.cpp

@@ -9,6 +9,10 @@
 #include <atomic>
 #include <memory>
 
+#if defined(WIN32)
+#include <Windows.h>
+#endif
+
 using namespace mbgl;
 using namespace mbgl::util;
 
@@ -198,7 +202,11 @@ TEST(Thread, ReferenceCanOutliveThread) {
         worker.invoke(&TestWorker::send, [&] { ADD_FAILURE() << "Should never happen"; });
     }
 
+#if defined(WIN32)
+    Sleep(10);
+#else
     usleep(10000);
+#endif
 }
 
 TEST(Thread, DeletePausedThread) {