Settings menu: display settings, language codes, style guide window

Display tab: window mode (windowed/maximized/borderless/exclusive), monitor, window
size per monitor (centred, disabled in fullscreen), V-Sync, frame rate cap, reset;
applied and saved at once through the core DisplaySettingsService. Language is read
through LanguageCode (en-en/en_US = English). The window follows the style guide
shell: title and tabs in the header, close on the right, footer hint and per-tab reset.
The main menu hides every session panel of UiHost (pause menu stayed above it).
New suite display_settings (+ --shots); collection_edit leaves the game through the
pause menu and checks it is closed.

manifest.json

@@ -14,6 +14,7 @@
     {"id": "save_dormancy", "activate": ["--itest=save_dormancy"]},
     {"id": "new_game", "activate": ["--itest=new_game"]},
     {"id": "hot_reload", "activate": ["--itest=hot_reload"]},
-    {"id": "collection_edit", "activate": ["--itest=collection_edit"]}
+    {"id": "collection_edit", "activate": ["--itest=collection_edit"]},
+    {"id": "display_settings", "activate": ["--itest=display_settings"]}
   ]
 }