WIP: Windows: Use Qt 5.6 for builds

Signed-off-by: Lukas Holecek <hluk@email.cz>

appveyor.yml

@@ -2,17 +2,13 @@
 configuration: Release
 
 image:
-- Visual Studio 2019
+- Visual Studio 2017
 
 environment:
   matrix:
-    - QTDIR: C:\Qt\5.12.6\mingw73_32
+    - QTDIR: C:\Qt\5.6\mingw49_32
       CMAKE_GENERATOR: MinGW Makefiles
-      MINGW_PATH: C:\Qt\Tools\mingw730_32
-
-    - QTDIR: C:\Qt\5.12.6\mingw73_64
-      CMAKE_GENERATOR: MinGW Makefiles
-      MINGW_PATH: C:\Qt\Tools\mingw730_64
+      MINGW_PATH: C:\Qt\Tools\mingw492_32
 
 # Parameters for default build commands (build_script is used instead).
 build: