Delay creating input devices to when required

Every first call to CInputManager::instance() was automatically creating
the low level input devices. This was not always desired and therefore
creation is now explicit via function.
Unit tests do not need the devices created. In contrast, Windows unit tests
failed since Windows didn't like the Jenkins service childs to allocate
DInput devices.

T391

src/blackcore/inputmanager.h

@@ -87,6 +87,9 @@ namespace BlackCore
         //! Triggers a key event manually and calls the registered functions.
         void triggerKey(const BlackMisc::Input::CHotkeyCombination &combination, bool isPressed);
 
+        //! Creates low level input devices. Once completed, hotkeys start to be processed.
+        void createDevices();
+
         //! Releases all devices
         void releaseDevices();