In the course of #5387, this patch splits the different logical pieces of the loading screen into individual classes to be extended in the future (for example D1730).
Style changes:
- New user config option to display the loading screen description. This may be extended in the future to display more information, even to interested players. The alternative would be to delete the feature in JS and C++, but it seems useful.
- Tip display receives functions that may be reused for D1730.
- The tip coordinates were recomputed to be harmony with / comparable with the other size values.
- One JS class and one GUI object XML file per page element with the filenames indicating the classname.
- Use translation for the percent value format