Bump the minimum Qt version to 5.3

There was a signal-slot connection bug not allowing
using pointers to member functions

Author: paceholder

CMakeLists.txt

@@ -34,7 +34,7 @@ endif()
 add_subdirectory(external)
 
 # Find the QtWidgets library
-find_package(Qt5 COMPONENTS
+find_package(Qt5 5.3 COMPONENTS
              Core
              Widgets
              Gui