UWP implementation

This merge request implements UWP for Windows and Xbox, among some changes within the way the video is displayed and the assets and writable paths are created/read, there are functions to stop and reload a quest initializing and deinitializing the resources, cleaning texture cache, fonts, and sprites. There is also mutex protection for the main loop enabled only for WinRT since the XAML enabled app makes use of multithreading for the purpose of running the game. Many warnings were fixed for this merge request

The outer shell of the project, solarus-uwp, contains a dummy quest used to run the main engine, since there is no default main arguments the quest must be changed by the developer. This is for the Core UWP project, the XAML project provides an interface to load and close quests with a nice Solarus image background. There are plans to extend the functionality to provide input configuration and video options (antialiazing, scale, among others). Just to note that Core UWP apps support OpenGL via Angle, whilst XAML only support DirectX 11, the plan is to continue development and patch SDL to implement OpenGL for XAML