GUI: welcome screen for first run

XMage must show welcome screen on first run. It can contains some fast settings and usage hints. Also that screen can show problem hints like missing images and symbols, paths, files access, etc.

Can contains:
* [ ] :information_source: Welcome message with resources and links like github issues, wiki, etc;
* [ ] :information_source: Images render mode info (IMAGE or MTGO);
* [ ] :hammer_and_wrench: Fast switch between modes;
* [ ] :information_source: Themes and font settings info;
* [ ] :hammer_and_wrench: Fast switch between predefined modes (example: default, for big screen, for small screen);
* [ ] :information_source: Images and symbols download info;
* [ ] :information_source: Warning message on missing images or symbols (check any standard image like mountain and mana icon availability). With buttons to open download dialogs;
* [ ] :information_source: Memory settings;
* [ ] :hammer_and_wrench: Fast text box to setup memory usage (change launcher settings file with carefully);
* [ ] :information_source: Warning message on wrong app's path (app in download folder, app's path contains unsupported symbols, etc);
* [ ] :information_source: How to play with AI via a local server;

Other:
* [ ] Checkbox to not show it again on startup;
* [ ] Main menu or toolbar button to show welcome dialog again;
* [ ] Auto-show it again on new problems found (example: check sample image availability on startup).